JP4032189B2 - Method for producing layered crystal structure oxide - Google Patents

Description

【００８４】
また、回折斑点の消滅則より、結晶１および結晶２の結晶空間群は、双晶が存在しないという仮定の元ではＰｎｎｍ，Ｐｎｎ２のいずれかにしぼられ、双晶が存在するという仮定の元ではＣｍｃ２₁ ，Ｃｍｃｍ，Ｃ２ｃｍの何れかにしぼられることが分かった。ここでは、ＮｅｗｎｈａｍやＲａｅらによる解析結果（R. E. Newnham, R. W. Wolfe, R. S. Horsey, F. A. Diaz-Colon and M. I. Kay, Mater. Res. Bull., 8 (1973) 1183. ； A. D. Rae, J. G. Thompson and R. L. Withers, Acta. Cryst., B48 (1992) 418. ）を初期モデルとし、これらの各々について回折強度分布を統計的に解析し、最終的には結晶１および結晶２とも双晶が存在し結晶空間群はＣｍｃ２₁ であると決定した。
【００８５】
なお、この空間群は“International Tables for Crystallography”に掲載された標準的な設定である。習慣的に良く用いられるｃ軸を長軸とする設定ではＡ２₁ ａｍであり、これは空間対称性としてはＮｅｗｎｈａｍやＲａｅらの解析結果と一致している。
【００８６】
また、ｂ軸の格子定数およびｃ軸の格子定数は、結晶１および結晶２ともリファインの結果からは精度上以上の差は観測されなかったが、空間対称性よりこれらが等価でなはいと結論付けることができる。これを裏付けるために、結晶１および結晶２についてクロスニコル状態下で偏光顕微鏡観察をそれぞれ行った。観察面はａ面である。その結果、結晶１および結晶２とも周期的な明暗反転が観察され、明らかにｂ軸とｃ軸の格子定数は等しくないということが確認された。
【００８７】
次いで、結晶構造の精密化を、解析ソフト“ＳＡＰＩ９１”（Fan Hai-Fu(1991). Structure Analysis Programs With Intelligent Control, Rigaku Corporation, Tokyo, Japan. ）を用いてパターソン法による最小自乗リファインメントでそれぞれ行った。表２に、結晶１の原子座標（双晶を前提とし元素の部分置換を仮定しない初期的な解析によるもの）を示す。その結果、図１に示した基本結晶構造（空間群；Ｉ４／ｍｍｍ）から原子サイトの変位が認められ、結晶がｃ軸方向に自発分極を有することが分かった（空間群；Ｃｍｃ２₁ ）。なお、このリファインメントによる信頼性因子（Ｒ−ｆａｃｔｏｒ）は３．７％であった。
【００８８】
【表２】

【００８９】
更に、この信頼性因子を収束させるために、結晶１についてフルオライト層１１中のビスマスとペロブスカイト層１２中のストロンチウムとが部分的に相互置換したモデルを立てて結晶構造の精密化を行った。表３にその結果を示す。表３において、Ｂｉ（２）はストロンチウムサイトＳｒ（２）に存在するビスマスであり、Ｓｒ（１）はビスマスサイトＢｉ（１）に存在するストロンチウムである。この結果から、結晶１ではビスマスサイトで５％、ストロンチウムサイトで２％程度のビスマスとストロンチウムとの相互置換が認められた。信頼性因子も、３．５％と僅かではあるが改善した。
【００９０】
【表３】

【００９１】
部分的に相互置換したモデルにおいては精密化パラメータの数が多いので、数学的に信頼性因子が改善されることは当然であると言えるが、Ｘ線の重元素に対する散乱能に問題はなく、また信頼性因子の絶対値は統計的にこのモデルが正しいことを示唆しうる程度であるので、この結晶１においては僅かではあるがビスマスとストロンチウムとが部分置換していると考えられる。
【００９２】
この結果に基づき結晶１のビスマスとストロンチウムとタンタルとの組成比を求めたところ、次のようになった。
結晶１ Ｂｉ：Ｓｒ：Ｔａ ＝ １．９２：１．０９：２
【００９３】
また、結晶２については、元素の部分置換を仮定しないモデルではシミュレーションデータと実測された回折データとの間の相違が統計的に大き過ぎ、信頼性因子は収束に至らなかった。そこで、信頼性因子を収束させるため、結晶１と同様に、フルオライト層１１中のビスマスとペロブスカイト層１２中のストロンチウムとが部分的に相互置換したモデルを立てて結晶構造の精密化を行った。表４にその結果を示す。表３においても、Ｂｉ（２）はストロンチウムサイトＳｒ（２）に存在するビスマスであり、Ｓｒ（１）はビスマスサイトＢｉ（１）に存在するストロンチウムである。この結果から、結晶２ではビスマスサイトで４２％、ストロンチウムサイトで２６％という顕著なビスマスとストロンチウムとの相互置換が認められた。信頼性因子は３．２％であった。信頼性因子が信頼性のあるレベルに収束していることから、結晶２においては非常に大きくビスマスとストロンチウムとが部分置換していると考えられる。
【００９４】
【表４】

【００９５】
この結果に基づき結晶２のビスマスとストロンチウムとタンタルとの組成比を求めたところ、次のようになった。
結晶２ Ｂｉ：Ｓｒ：Ｔａ ＝ １．４３：１．５８：２
【００９６】
これらの結果から、このビスマスとストロンチウムとの部分置換に伴って起こる結晶構造全体の結晶化学的変化について次のように考察することができる。すなわち、Ｂｉ³⁺の存在する８ｂ−ｓｉｔｅはＳｒ²⁺の存在する４ａ−ｓｉｔｅに対して多重度が２倍であるので、これらのサイトで相互置換が生ずると、Ｂｉ³⁺が減少しＳｒ²⁺が増加する傾向を示す。その結果、結晶中の正電荷が減少することになり、電荷中性を保つためには酸素欠陥が必要となる。
【００９７】
図１から分かるように、この層状結晶構造酸化物には異なる５つの酸素サイトがあり、ＳｒＯ面内の酸素Ｏ（１）、フルオライト層１１とペロブスカイト層１２との境界に位置する酸素Ｏ（２）、フルオライト層１１中の酸素Ｏ（３）、ＴａＯ₂ 面内の酸素Ｏ（４），Ｏ（５）とそれぞれ記述することができる。なお、Ｏ（４）とＯ（５）は化学的環境が極めて似通っているが、格子歪みに伴う対称性低下（Ｆ４／ｍｍｍからＣｍｃ２₁ ）により区別される。ここで表４を見てみると、これらの酸素のうちまずフルオライト層１１中の酸素Ｏ（３）の充填率のみが極めて小さくなっており、このＯ（３）サイトの酸素が選択的に欠損し易いことが分かる。これはフルオライト相が有している大きな不定比性からも十分考えられる。
【００９８】
更に特徴的なことは、Ｏ（２）サイトがＯ（２）とＯ（２’）という充填率がほぼ５０％づつの２つのサイトに分離していることである。しかし、現実的には最近接しているＯ（２）サイトとＯ（２’）サイトが同時に占有されることはクローン反発が大きくなるのでまずあり得ず、ディスオーダー（disorder）により最近接のＯ（２）サイトとＯ（２’）サイトのいずれかが占められていると考えられる。
【００９９】
なお、このような欠陥構造はある程度は普遍性があると思われるが、基本的には本実施例において認められたビスマスとストロンチウムとの相互部分置換に起因すると考えられ、組成変化によるストロンチウムサイトのビスマスによる一方的部分置換や、異なったサイト間の置換など欠陥構造は種々変化するものと考えられる。
【０１００】
以上の結果に基づき格子歪みまで含めて忠実に再現した結晶１の結晶構造を図１５に示す。同様に再現した結晶２の結晶構造を図１６に示す。図１５および図１６においては、ａ軸方向に１ユニットセル、ｂ軸ｃ軸方向に２ユニットセルを示している。ちなみに、煩雑さを避けるためにＴａＯ₆ の構造を八面体で示したが、境界領域において完全な八面体が形成されない部分では、タンタルと酸素をそれぞれ原子として示している。
【０１０１】
また、図１５（すなわち結晶１）をｂ軸方向から見た結晶構造を図１７に示し、ｃ軸方向から見た結晶構造を図１８に示す。同様に、図１６（すなわち結晶２）をｂ軸方向から見た結晶構造を図１９に示し、ｃ軸方向から見た結晶構造を図２０に示す。図１７および図１８から、結晶１では、フルオライト層１１の酸素平面はほぼ平坦であること、ＴａＯ₆ の八面体におけるＴａＯ₂ 面の傾きはほぼ分極軸であるｃ軸の周りに生じている（すなわち回転軸がｃ軸である）ことが分かる。また、図１９および図２０から、結晶２では、フルオライト層１１の酸素平面が曲がっていることが分かる。なお、結晶２では、酸素サイトに相当欠陥が存在すること、およびフルオライト層１１との境界の頂点酸素サイトが分離しているのは上述のとおりである（ここではタンタルとの原子間距離がより小さいＯ（２’）を頂点として八面体を表している）。
【０１０２】
更に、図１７乃至図２０から、結晶１および結晶２ではペロブスカイト層１２の分極もさることながらフルオライト層１１においてもビスマスと酸素の相対変位が決して小さくなく、それらが分極に寄与していると考えられる。実際に格子歪みのない母体構造（Ｆ４／ｍｍｍ）を基準としたフルオライト層１１中の陽イオン（ビスマス、またはビスマスサイトに置換しているストロンチウム）と陰イオン（酸素Ｏ（３））との相対変位の分極軸成分を算出すると、分極軸（ｃ軸）の単位格子長（Ｃ₀ ≒５．５Å）に対して結晶１で約８．９７％，結晶２で約５．４７％となる。一般には、分極を担うのは主としてペロブスカイト層１２であると考えられており、このように大きな分極がフルオライト層１１中で生じていることが確認された例はない。
【０１０３】
ちなみに、ＮｅｗｎｈａｍやＲａｅらによる解析結果（R. E. Newnham, R. W. Wolfe, R. S. Horsey, F. A. Diaz-Colon and M. I. Kay, Mater. Res. Bull., 8 (1973) 1183. ； A. D. Rae, J. G. Thompson and R. L. Withers, Acta. Cryst., B48 (1992) 418. ）におけるフルオライト層中の陽イオンと陰イオンとの相対変位の分極軸成分は、それぞれ２．８０％および１．６９％である。
【０１０４】
なお、ここで分極軸の単位格子長は通常のフルオライト相と同様に定義し、約５．５Åとした。しかしながら、例えば分極軸またはこれを含むｂｃ面，ｃａ面に長周期構造が存在する場合には、格子定数が大きくなり、この相対変位の分極軸成分の計算値は見かけ上小さくなる。従って、そのような場合には、通常のフルオライト相に相当する単位格子長に対して相対変位の分極軸成分を見積もるようにする。
【０１０５】
また、結晶１のうちフルオライト層１１およびこれに隣接する頂点酸素を抽出しａ軸方向から見た結晶構造を図２１に示し、同様に結晶２のうち同様にフルオライト層１１およびこれに隣接する頂点酸素を抽出しａ軸方向から見た結晶構造を図２２に示す。更に、結晶１のうちペロブスカイト層１２のＴａＯ₆ 八面体およびこれに隣接するストロンチウムを抽出しａ軸から見た結晶構造を図２３に示し、同様に結晶２のうちペロブスカイト層１２のＴａＯ₆ 八面体およびこれに隣接するストロンチウムを抽出しａ軸から見た結晶構造を図２４に示す。
【０１０６】
図２１および図２２から、結晶１ではほぼ平坦で歪みの少ないフルオライト層１１中の酸素Ｏ（３）の結合が、結晶２では酸素欠陥によりｂｃ面の平坦性が損なわれていると共にｂｃ面内でも大きく歪んでいることが分かる。また、結晶２に認められてる頂点酸素のディスオーダーは、分極軸方向（ｃ軸方向）またはそれに垂直な方向（ｂ軸方向）ではなく、（ｂ＋ｃ）方向に近いことが分かる。
【０１０７】
更に、図２３および図２４から、結晶１では、上述したように、ＴａＯ₂ 面の傾きはほぼ分極軸であるｃ軸の周りに生じているが、頂点酸素に関してはややｂ軸回転の成分も有していることが分かる。一方、結晶２では、頂点酸素のディスオーダーと相まってＴａＯ₂ の結合がｂｃ面内で歪んでいることが分かる。
【０１０８】
以上より、この層状結晶構造酸化物は、ビスマスとストロンチウムとが部分置換し、またはフルオライト層１１を構成する酸素が選択的に欠損しあるいはフルオライト層１１とペロブスカイト層１２との境界に位置する頂点酸素がディスオーダーし、またはフルオライト層１１の陽イオンと陰イオンとが互いに分極軸方向に相対変位していることにより、特有の常誘電性あるいは強誘電性を示すことが分かった。
【０１０９】
以上、実施の形態および各実施例を挙げて本発明を説明したが、本発明はこれらの実施の形態および実施例に限定されるものではなく種々変形可能である。例えば、上記実施例においては、化学量論的な組成式がＢｉ₂ Ｓｒ₁ Ｔａ₂ Ｏ₉ である層状結晶構造酸化物に関し説明したが、本発明は、化１において示した化学式で代表されるこれ以外のいわゆるオーリビリウス結晶群の層状結晶構造酸化物についても同様の結果を得ることができる。
【０１１０】
具体的に説明すれば、化１におけるｍが２のＢｉ₂ ＢｉＴｉＮｂＯ₉ やＢｉ₂ ＢｉＴｉＴａＯ₉ の化学式により代表される層状結晶構造酸化物、ｍが３のＢｉ₄ Ｔｉ₃ Ｏ₁₂の化学式により代表される層状結晶構造酸化物、ｍが４のＢｉ₂ Ｂｉ₂ ＣａＴｉ₄ Ｏ₁₅ ，Ｂｉ₂ Ｂｉ₂ ＳｒＴｉ₄ Ｏ₁₅ ，Ｂｉ₂ Ｂｉ₂ ＢａＴｉ₄ Ｏ₁₅ ，Ｂｉ₂ Ｂｉ₂ ＰｂＴｉ₄ Ｏ₁₅ ，Ｂｉ₂ Ｂｉ_2.5 Ｎａ_0.5 Ｔｉ₄ Ｏ₁₅やＢｉ₂ Ｂｉ_2.5 Ｋ_0.5 Ｔｉ₄ Ｏ₁₅の化学式により代表される層状結晶構造酸化物、ｍが５のＢｉ₂ Ｂｉ₂ Ｓｒ₂ Ｔｉ₅ Ｏ₁₈ ，Ｂｉ₂ Ｂｉ₂ Ｂａ₂ Ｔｉ₅ Ｏ₁₈やＢｉ₂ Ｂｉ₂ Ｐｂ₂ Ｔｉ₅ Ｏ₁₈の化学式により代表される層状結晶構造酸化物についても本発明は同様に適用される。
【０１１１】
また、本発明は、いわゆるオーリビリウス結晶群のみでなく化３に示した化学式で表され超伝導材料として知られる層状結晶構造酸化物についても同様に適用される。
【化３】
Ｂｉ₂ Ｓｒ₂ Ｃａ_n-1 Ｃｕ_n Ｏ_2n+4 なお、ｎ≦４
【０１１２】
これは、いわゆるＢｉ系超伝導層状酸化物といわれるものであり、結晶構造がオーリビリウス結晶群と類似関係にある。特に、層間がＢｉ−Ｏで仕切られていることが異方性の点で酷似している。
【０１１３】
更に、上記実施の形態および実施例においては、層状結晶構造酸化物の製造においてフラックスを実質的に全て蒸発させるようにしたが、育成した層状結晶構造酸化物を取り出せる程度に蒸発させれば、上記実施の形態および実験例と同様の効果を得ることができる。
【０１１４】
加えて、上記実施の形態および実施例においては、層状結晶構造酸化物の製造においてフラックスを加えた原料を融点以上の所定温度に加熱してその所定温度で一定時間維持したのち自然放冷よりも遅い速度で徐冷することにより、融点以上の温度に所定時間保持し、フラックスを実質的に全て蒸発させるようにしたが、本発明は、他の方法により融点以上の温度に所定時間保持しフラックスを実質的に全て蒸発させるようにしてもよい。
【０１１５】
例えば、フラックスを加えた原料を融点以上の所定温度に加熱してその所定温度で一定時間維持することにより融点以上の温度に所定時間保持し、フラックスを実質的に全て蒸発させるようにしてもよい。また、フラックスを加えた原料を融点以上の所定温度に加熱したのち放冷しつつ融点以上の温度に所定時間保持し、フラックスを実質的に全て蒸発させるようにしてもよい。更に、フラックスを加えた原料を融点以上の所定温度に加熱したのち自然放冷よりも遅い速度で徐冷しつつ融点以上の温度に所定時間保持し、フラックスを実質的に全て蒸発させるようにしてもよい。
【０１１９】
【発明の効果】
本発明の層状結晶構造酸化物の製造方法によれば、フラックスを加えた原料を融点以上の温度に一定時間保持することにより、層状結晶構造酸化物を取り出すことができる程度にフラックスを蒸発させるようにしたので、育成した層状結晶構造酸化物を直接取り出すことができる。よって、層状結晶構造酸化物を取り出すための後工程が不要となると共に、酸によるエッチングなどによって層状結晶構造酸化物が浸食されることを回避することができるという効果を奏する。また、フラックスが少なくなると融点が高くなるので、製造に要する時間を短縮することができ、効率よく層状結晶構造酸化物を製造することができるという効果も奏する。
【図面の簡単な説明】
【図１】本発明の一実施の形態に係る層状結晶構造酸化物の基本的な結晶構造を表す概念図である。
【図２】図１に示した層状結晶構造酸化物を製造する際に用いる二重ルツボを表す構成図である。
【図３】図１に示した層状結晶構造酸化物を製造する際に用いる縦型シリコニット炉を表す構成図である。
【図４】図１に示した層状結晶構造酸化物を製造する際の熱処理における温度条件を表す図である。
【図５】本発明の第１の実施例における熱処理の際の温度条件を表す図である。
【図６】図５に示した温度条件により得られた一結晶の組成を示すＸ線回折パターン図（ａ）であり、リートベルト・シミュレーション・パターン（ｂ）と共に表している。
【図７】図５に示した温度条件における定温工程の維持時間により得られる結晶の組成の相違を説明するためのＸ線回折パターン図である。
【図８】図５に示した温度条件において得られた層状結晶構造酸化物のラウエ斑点を示す顕微鏡写真である。
【図９】図５に示した温度条件において得られた層状結晶構造酸化物に関する熱分析の結果を表す特性図である。
【図１０】図５に示した温度条件において得られた層状結晶構造酸化物の強誘電性ヒステリシスを表す特性図である。
【図１１】本発明の第１の実施例における熱処理の際の他の温度条件を表す図である。
【図１２】図１１に示した温度条件における定温工程の維持時間により得られる結晶の組成の相違を説明するためのＸ線回折パターン図である。
【図１３】本発明の第１の実施例における熱処理の際の更に他の温度条件を表す図である。
【図１４】図１３に示した温度条件における定温工程の維持時間により得られる結晶の組成の相違を説明するためのＸ線回折パターン図である。
【図１５】本発明の第２の実施例に係る層状結晶構造酸化物（結晶１）の結晶構造を格子歪みまで含めて表す概念図である。
【図１６】本発明の第２の実施例に係る層状結晶構造酸化物（結晶２）の結晶構造を格子歪みまで含めて表す概念図である。
【図１７】図１５に示した層状結晶構造酸化物（結晶１）のｂ軸から見た結晶構造を格子歪みまで含めて表す概念図である。
【図１８】図１５に示した層状結晶構造酸化物（結晶１）のｃ軸から見た結晶構造を格子歪みまで含めて表す概念図である。
【図１９】図１６に示した層状結晶構造酸化物（結晶２）のｂ軸から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２０】図１６に示した層状結晶構造酸化物（結晶２）のｃ軸から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２１】図１５に示した層状結晶構造酸化物（結晶１）のうちフルオライト層およびこれに隣接する頂点酸素を抽出しａ軸方向から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２２】図１６に示した層状結晶構造酸化物（結晶２）のうちフルオライト層およびこれに隣接する頂点酸素を抽出しａ軸方向から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２３】図１５に示した層状結晶構造酸化物（結晶１）のうちペロブスカイト層のＴａＯ₆ 八面体およびこれに隣接するストロンチウムを抽出しａ軸方向から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２４】図１６に示した層状結晶構造酸化物（結晶２）のうちペロブスカイト層のＴａＯ₆ 八面体およびこれに隣接するストロンチウムを抽出しａ軸方向から見た結晶構造を格子歪みまで含めて表す概念図である。
【図２５】フルオライト相の結晶構造を表す概念図である。
【図２６】ペロブスカイト相の結晶構造を表す概念図である。
【符号の説明】
１１…フルオライト層、１２…ペロブスカイト層、２０…二重ルツボ、２１…白金ルツボ、２２…アルミナルツボ、２３…蓋、３０…縦型シリコニット炉、３１…炉心管、３２…載置台、３３…ヒータ、３４…断熱材、３５…蓋、Ｍ…原料[0001]
BACKGROUND OF THE INVENTION
  The present invention provides a layered crystal structure oxidation of so-called Aurivirius crystals and superconducting materials.ThingIt relates to a manufacturing method.
[0002]
[Prior art]
The bismuth layered crystal structure oxide is a bismuth superconducting oxide having a critical temperature of 110K, or a bismuth layered ferroelectric material (C.A.) recently attracting attention as a material for a ferroelectric nonvolatile memory (FeRAM). -Paz de Araujo, JD Cuchiaro, LD McMillan, MC Scott and JF Scott, Nature, 374 (1995) 627 .; K. Amanuma, T. Hase and Y. Miyasaka, Appl. Phys. Lett., 66 (1995) 221 SB Desu and DP Vijay, Master. Sci. And Eng., B32 (1995) 75. In particular, bismuth strontium tantalate: Bi₂SrTa₂O₉(Hereinafter referred to as BiSTa) is excellent in fatigue resistance due to polarization reversal and can be driven at a low voltage, and is considered the most promising candidate capacitor material for FeRAM (CA Paz de Araujo, JD Cuchiaro, MC Scitt and LD McMillan, International Patent Publication No. WO 93/12542 (24 June 1993)).
[0003]
Recently, it has been reported that this BiSTa has been successfully prepared by MOCVD (Metal Organic Chemical Vapor Deposition) (T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama). , Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195 .; T. Li, Y. Zhu, SB Desu, CH. Peng, M. Nagata, Appl. Phys. Lett., 68 (1996) 616.).
[0004]
By the way, many discussions have been made so far regarding the relationship between the composition of BiSTa and the ferroelectricity, and a composition in which bismuth is slightly excessive and strontium is somewhat deficient, typically Bi._2.2Sr_0.8Ta₂O₉It is said that the best ferroelectricity can be obtained in the vicinity (T. Noguchi, T. Hase and Y. Miyasaka, Jpn. J. Appl. Phys. 34, 4900 (1996), etc.). For this reason, it has been speculated that partial substitution of bismuth and strontium may have occurred in this optimum composition.
[0005]
However, many of these discussions on composition are based on the composition of the starting material, and otherwise are based on polycrystalline thin films that may have grain boundaries and other by-product phases. There is a problem in data accuracy. For example, there is a report that the presence of an amorphous phase was confirmed by observation with a high-resolution transmission electron microscope (TEM) even though it was considered to be a single phase in X-ray analysis (CD Gutleben, Y Ikeda, C. Isobe, A. Machida, T. Ami, K. Hironaka and E. Morita, Mater. Res. Soc. Symp. Proc., 415 (1996) 201.). Therefore, it is completely unknown whether partial substitution of elements has occurred.
[0006]
When this is considered from the viewpoint of the crystal structure, this BiSTa is called a so-called Aurivirius crystal group.₂SrTa₂O₉The basic crystal structure (matrix structure; space group I4 / mmm or F4 / mmm) is as shown in FIG. By the way, the Aurivirius crystal group has the composition formula [Bi₂O₂]²⁺[Me_m-1R_mO_{3 m + 1}]^2-(M is an integer of 2 or more, Me is sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), bismuth (Bi ), At least one of the group consisting of iron (Fe), titanium (Ti), vanadium (V), niobium (Nb), tantalum (Ta), and tungsten (W). seed).
[0007]
As shown in FIG. 1, BiSTa has a perovskite structure (ABO) with strontium as the A site and tantalum as the B site._ThreeIt can be considered that a layer (perovskite layer) composed of two unit lattices) is sandwiched between layers having a fluorite structure (fluorite layer) composed of bismuth and oxygen. Of these, it is generally considered that the perovskite layer has ferroelectricity and the fluorite layer has a function of maintaining the crystal structure. The bismuth at the A site at the boundary of the perovskite layer is shared with the fluorite layer.
[0008]
However, the production process research by MOCVD method has revealed that this material has a large non-stoichiometry and can be produced by heat-treating the metastable fluorite phase shown in FIG. 25 as a crystal nucleus. (T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc , 415 (1996) 195.) In another study, the crystallinity of the fluorite phase was lowered by lowering the temperature of the production process, etc. It has been confirmed that the layered crystal structure described above is obtained via this fluorite phase (C. Isobe et al., 9th Int'l Symp. On Integrated Ferroelectrics Abstr. & Program, 79i). In this case, since there is only one metal site in the fluorite phase, it is considered that all metal elements of bismuth, strontium, and tantalum occupy an equivalent site over a wide composition range.
[0009]
In the fluorite phase, for example, in a partially stable zirconia, a considerably large amount of an alkaline earth metal or the like can be dissolved in a zirconium (Zr) site, and an oxygen-rich non-stoichiometric compound can be formed in uranium oxide. As can be seen, non-stoichiometry exists. This non-stoichiometry naturally applies to defects, and both cations and anions can have a large defect density. In fact, in this fluorite structure, the anion should ideally be 8-coordinated around the cation, but δ-Bi₂O_ThreeIn some cases, the oxygen coordination number of bismuth is reduced to about 6 due to oxygen defects while maintaining this structure (ICPDS, 16-654; d-Bi₂O_ThreeGattow and Schroder, Z. Anorg. Allgem. Chem. 318 (1962), 176). In this case, among the oxygen atoms that form a regular hexahedron in 8-coordinate, two oxygen atoms diagonal to each other are selectively lost, and the remaining oxygen atoms form a distorted octahedron around bismuth. is doing. Because of these properties, the fluorite phase is also tolerant to lattice strain and non-uniform strain in the crystal, and typically has a symmetry from tetragonal to monoclinic at room temperature due to lattice strain in zirconia. It is not uncommon for oxygen to take 7-coordinates and the half-width of the X-ray diffraction peak by θ-2θ scanning reaches several degrees or more due to non-uniform strain.
[0010]
On the other hand, although not as much as the fluorite phase, the perovskite phase shown in FIG. 26 has some non-stoichiometry. Examples of non-stoichiometry at the A site include oxide superconducting materials such as La₂CuO_FourThere are many lanthanum sites with barium, strontium and potassium added. Examples of non-stoichiometry of B site include BaBiO_ThreeBismuthsite with lead added.
[0011]
Thus, from the viewpoint of the generation mechanism, this substance can be said to be “a structure in which metal atoms are ordered by heat treatment from a highly stable metastable fluorite phase”. However, considering that the fluorite structure is partly retained in the structure itself and that the perovskite part also has a certain degree of non-stoichiometry, this ordering always proceeds in perfect form. Is hard to think. This is especially true when there is a composition shift.
[0012]
Therefore, in order to discuss such composition and crystal chemistry aspects, a single crystal is extremely important as a sample. For single crystals of the Aurivirius crystal group, Bi_FourTi_ThreeO₁₂(M = 3; Me = Bi; R = Ti) (J. F. Dorrian, R. E. Newnham, D. K. Smith and M. I. Kay, Ferroelectrics, 3 (1971) 17. etc.) and recently Bi_FourBaTi_FourO₁₅(M = 4; Me = Bi, Ba; R = Ti) (SK. Kim, M. Miyayama and H. Yanagida, J. Ceram. Soc, Japan, 102 (1994) 722. etc.) are being studied. . However, with BiSTa, there are very few reliable reports (RE Newnham, RW Wolfe, RS Horsey, FA Diaz-Colon and MI Kay, Mater. Res. Bull., 8 (1973) 1183 .; AD Rae, JG Thompson and RL Withers, Acta. Cryst., B48 (1992) 418.). Moreover, of the two papers related to BiSTa, those by Newham lacked precision such as composition handling, and those by Rae started with a stoichiometric composition, but a plate-like single crystal was obtained only in a two-phase mixed state. It has not yet been synthesized as a single phase. Furthermore, almost no analysis has been made on the properties of the single crystals obtained in both cases.
[0013]
That is, it is very important to produce a single crystal of the Aurivirius crystal group and clarify the crystal chemical aspect thereof when applying it to FeRAM as a material exhibiting ferroelectricity. It is also suggested that these Aurillius crystals may exhibit paraelectric properties depending on their composition or crystallinity (Tatsuo Machida, Naomi Nagasawa, Takaaki Ami, Masayuki Suzuki, Fall 1995, 57th JSAP [9p-F-2]), which has important implications for new DRAM materials and for use in new stacked devices.
[0014]
As for the preparation of single crystals of the Aurivirius crystal group, the self-flux method (Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T. Murakami) used in the preparation of high-temperature superconducting oxides was used. , J. Cryst. Growth, 85 (1987) 581 .; Y. Hidaka, M. Oda, M. Suzuki, Y. Maeda, Y. Enomoto and T. Murakami, Jpn, J. Appl. Phys., 27 (1988 ) (See L538.) (M. Suzuki, N. Nagasawa, A. Machida and T. Ami, Jpn, J. Appl. Phys., 35 (1996) L564.). By the way, the flux method is a method in which a melt is prepared by appropriately adding a flux (flux) to a raw material and heating, and this is gradually cooled to generate a target crystal nucleus and grow a crystal. is there. In this flux method, normally, the flux remains even after the crystal is grown, and it is necessary to separate the grown crystal from the flux.
[0015]
However, when this Aurivirius crystal group is produced by the self-flux method, for example, even if it is attempted to remove the flux by washing with hot water, it cannot be removed because the flux does not dissolve in water, and an acid such as hydrochloric acid (HCl) can be removed. Even if an attempt is made to remove the flux by etching, the crystal grown together with the flux is also eroded, so that there is a problem that it is difficult to take out the crystal.
[0017]
[Problems to be solved by the invention]
  The present invention has been made in view of such background and problems, and its purpose is to show ferroelectricity and paraelectricity.It is an object of the present invention to provide a method for producing a layered crystal structure oxide capable of easily producing a layered crystal structure oxide.
[0021]
[Means for Solving the Problems]
  The method for producing a layered crystal structure oxide of the present invention is a method for producing a layered crystal structure oxide using a self-flux method, wherein the raw material to which the flux is added is heated to a temperature equal to or higher than the melting point, and then the melting point or higher. By holding at this temperature for a certain period of time, the flux is evaporated to such an extent that the layered crystal structure oxide can be taken out to produce the layered crystal structure oxide.
[0023]
In the method for producing a layered crystal structure oxide of the present invention, a flux is added to a raw material and heated to a temperature equal to or higher than the melting point, and held at a temperature equal to or higher than the melting point for a certain time. Thereby, the flux evaporates to such an extent that the layered crystal structure oxide can be taken out, and the layered crystal structure oxide can be taken out.
[0024]
Embodiment
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0025]
The layered crystal structure oxide according to the present embodiment is composed of bismuth, a first element Me, a second element R, and oxygen. The first element is at least one member selected from the group consisting of sodium, potassium, calcium, barium, strontium, lead and bismuth, and the second element is selected from the group consisting of iron, titanium, vanadium, niobium, tantalum and tungsten. At least one of them. The first element Me is preferably at least one selected from the group consisting of strontium, lead, barium and calcium, and the second element R is preferably at least one selected from the group consisting of niobium and tantalum. Furthermore, the first element Me is preferably at least one member selected from the group consisting of strontium and calcium. In particular, the first element Me is preferably strontium and the second element R is preferably tantalum.
[0026]
Chemical formula 1 shows the stoichiometric composition formula of this layered crystal structure oxide. However, the layered crystal structure oxide according to the present embodiment is not limited to the stoichiometric one, but includes those that deviate from the stoichiometric composition.
[0027]
[Chemical 1]
[Bi₂O₂]²⁺[Me_m-1R_mO_{3m + 1}]^2-
Me; first element
R: second element
m: any integer of 2, 3, 4 or 5
[0028]
In addition, FIG. 1 shows a basic crystal structure of a layered crystal structure oxide in which m in the composition formula shown in Chemical Formula 1 is 2. Thus, this layered crystal structure oxide is [Bi₂O₂]²⁺And a fluorite layer 11 corresponding to_m-1R_mO_{3m + 1}]^2-Perovskite layers 12 corresponding to the above are alternately laminated and have an anisotropic a-axis cleavage (H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. Appl. Phys., 27 (1988) L209 .; see K. Hiraga, M. Hirabayashi, M. Kikuchi and Y. Syono, Jpn. J. Appl. Phys., 27 (1988) L573.).
[0029]
Although not shown in FIG. 1, the layered crystal structure oxide may be partially substituted with at least two elements at different lattice positions. For example, bismuth, the first element Me, and the second element R may be partially substituted with each other, or bismuth and the first element Me may be partially substituted with each other.
[0030]
Further, although not shown in FIG. 1, in this layered crystal structure oxide, at least one element may be partially deficient or disordered. For example, oxygen constituting the fluorite layer 11 may be selectively lost or disordered, or apex oxygen located at the boundary between the fluorite layer 11 and the perovskite layer 12 may be disordered.
[0031]
Further, although not shown in FIG. 1, this layered crystal structure oxide is composed of the cation (that is, bismuth or bismuth site of the first element Me or the second element R substituted for the fluorite layer 11). ) And anions (ie oxygen) are unit cell lengths (C₀ In some cases, the relative displacement may be 3% or more (approximately 5.5 mm). Ferroelectricity is determined by the relative displacement between the cation and the anion in the entire crystal, but in general with respect to the fluorite layer that is considered not to contribute much to the ferroelectricity, It is considered that the larger the relative displacement, the better. For example, 5% or more is preferable, and 7% or more is more preferable.
[0032]
In addition, the layered crystal structure oxide includes a paraelectric property and a ferroelectric property in the c-axis direction. This is considered to be due to the difference in crystallinity, or partial substitution of elements, partial defects, or displacement.
[0033]
As described above, according to the layered crystal structure oxide according to the present embodiment, at least two elements at different lattice positions are partially substituted, or at least one element is partially missing or disordered. Therefore, a substance exhibiting ferroelectricity or paraelectricity can be easily obtained as compared with a completely crystalline material. Therefore, those exhibiting ferroelectricity can be easily applied to FeRAM, etc., and those exhibiting paraelectricity can be used as auxiliary substances for forming capacitors as a new substance exhibiting paraelectric properties and for DRAMs. It can be used as a capacitor material.
[0034]
In addition, according to the layered crystal structure oxide according to the present embodiment, the cation and the anion of the fluorite layer 11 are relatively displaced from each other by 3% or more of the unit cell length in the polarization axis direction. Can be obtained. Therefore, it can be applied to FeRAM and the like.
[0035]
The layered crystal structure oxide having such a configuration can be manufactured as follows. Here, the method for producing the layered crystal structure oxide of the present invention is also described.
[0036]
First, an appropriate oxide raw material is mixed at an appropriate ratio. For example, the stoichiometric composition formula is Bi₂Sr₁Ta₂O₉When the layered crystal structure oxide is produced, the raw material is bismuth oxide (Bi₂O_Three) And strontium carbonate (SrCO_Three) And tantalum oxide (Ta₂O_FiveAnd bismuth oxide 45-80, strontium carbonate 10-15, and tantalum oxide 10-15 in a molar ratio. Preferably, bismuth oxide 46.6 to 79.0, strontium carbonate 10.5 to 14.3, tantalum oxide 10.5 to 14.3, and most preferably bismuth oxide 79.0, strontium carbonate 10.5. , Tantalum oxide 10.5.
[0037]
Here, since bismuth oxide is used as a flux, the molar ratio of bismuth oxide is high. Thus, when producing a layered crystal structure oxide containing bismuth, it is particularly preferable to use bismuth oxide as a flux.
[0038]
Next, a raw material to which bismuth oxide as a flux is added is placed in a suitable crucible, and is inserted into a suitable furnace and subjected to heat treatment.
[0039]
FIG. 2 shows the structure of the double crucible 20 used in the present embodiment. The double crucible 20 includes a platinum crucible 21 into which the raw material M is put and an alumina crucible 22 into which the platinum crucible 21 is put. An alumina lid 23 is placed on the alumina crucible 22. The reason for melting the raw material M using the double crucible 20 is to control the evaporation of the bismuth compound.
[0040]
FIG. 3 shows the configuration of the vertical siliconite furnace 30 used in the present embodiment. The vertical silicon furnace 30 includes a vertical core tube 31, and the double crucible 20 mounted on the mounting table 32 is inserted in the vicinity of the center of the core tube 31. ing. The core tube 31 is made of, for example, alumina (Al₂O_Three) And mullite.
[0041]
A heater 33 is disposed in the vicinity of the outer periphery of the core tube 31 and the outer periphery thereof is covered with a heat insulating material 34, and a soaking part A is formed in the vicinity of the double crucible 20. The temperature of the soaking part A can be measured by a thermocouple 35 disposed in the vicinity of the soaking part A. The volume in the core tube 31 in the soaking part A is 5 to 30 times the volume of the double crucible 20. This volume ratio is important in controlling the evaporation of the bismuth compound.
[0042]
Further, a lid 36 is disposed on the upper portion of the core tube 31, and a forced exhaust device (not shown) for exhausting the bismuth compound is disposed in the vicinity thereof. In other words, the core tube 31 serves as an exhaust path for guiding the evaporated bismuth compound to a forced exhaust device (not shown) while preventing the bismuth compound evaporated from the double crucible 20 from adhering to the heater 33. Have.
[0043]
FIG. 4 shows the temperature conditions when the vertical silicon furnace 30 is heat-treated. As described above, in the present embodiment, first, the raw material M is heated to a melting point or higher (for example, 1330 ° C. or higher and 1450 ° C. or lower) at an appropriate temperature rising rate (heating step), and then maintained at this heating temperature for a predetermined time. (Constant temperature process) Subsequently, it is gradually cooled to a temperature of, for example, 800 ° C. or higher and 1300 ° C. or lower (slow cooling process) at a slower rate than natural cooling (for example, 1 ° C./hour to 20 ° C./hour or less). That is, by this, the raw material to which bismuth oxide as a flux is added is kept at a temperature equal to or higher than the melting point for a predetermined time, and substantially all of the flux is evaporated. Then, it cools to room temperature by natural cooling (natural cooling process). In the example shown in FIG. 4, in the heating step, the bismuth oxide has a melting point of 1330 ° C. or higher.
[0044]
Furthermore, if some preferable specific examples regarding this heat treatment are described, for example, the raw material M is heated to 1350 ° C. or higher and 1450 ° C. or lower at an appropriate temperature rising rate (heating step), and then at this heating temperature for more than 1 hour. It is maintained for a longer time than 3 hours (constant temperature step), and then gradually cooled to a temperature of 800 ° C. to 1300 ° C. at a slow cooling rate of 1 ° C./hour to 5 ° C./hour (slow cooling step). Then, it cools to room temperature by natural cooling (natural cooling process).
[0045]
Further, for example, the raw material M is heated to 1350 ° C. or higher and 1450 ° C. or lower at an appropriate temperature rising rate (heating step), and then maintained at this heating temperature for a time longer than 10 hours and shorter than 40 hours (a constant temperature step), Subsequently, it is gradually cooled to a temperature of, for example, 800 ° C. to 1300 ° C. at a slow cooling rate of 5 ° C./hour to 20 ° C./hour (slow cooling step). Then, it cools to room temperature by natural cooling (natural cooling process).
[0046]
Also, for example, the raw material M is heated to 1330 ° C. or higher and 1370 ° C. or lower at an appropriate temperature rising rate (heating step), and then maintained at this heating temperature for a time longer than 40 hours and shorter than 50 hours (constant temperature step), Subsequently, it is gradually cooled to a temperature of 800 ° C. or higher and 1300 ° C. or lower at a slow cooling rate of 1 ° C./hour or more and 5 ° C./hour or less (slow cooling step). Then, it cools to room temperature by natural cooling (natural cooling process).
[0047]
Here, an impurity layer is generated if the maintenance time in the constant temperature process is too long or too short. For example, the stoichiometric composition formula is Bi₂Sr₁Ta₂O₉When the layered crystal structure oxide is produced, if the maintenance time in the low temperature process is too long, SrTa_FourO₁₁(JCPDS No.16-708) and Sr₂Ta₂O₇If the impurity phase of (JCPDS No. 30-1304) is generated and the maintenance time is too short, β-Bi₂O_Three(JCPDS No.18-224) or α-Bi₂O_Three(JCPDS No.27-53) or δ-Bi₂O_ThreeThe impurity phase (JCPDS No. 27-52) is generated. Pure single crystal color is colorless and transparent, but SrTa_FourO₁₁When the impurity phase is formed, the crystal turns white, and β-Bi₂O_ThreeOr α-Bi₂O_ThreeWhen the impurity phase is formed, the crystal becomes light yellow.
[0048]
As a result, a layered crystal structure oxide is manufactured, and substantially all of the flux is evaporated, and the layered crystal structure oxide can be taken out. Note that the layered crystal structure oxide obtained by the method according to this embodiment is mainly represented by the chemical formula shown in Chemical Formula 2.
[Chemical formula 2]
Bi_2-aSr_{1 + b}Ta₂O_{9 + c}
0 ≦ a ≦ 0.14, 0 ≦ b ≦ 0.35, −0.21 ≦ c ≦ 0.35
[0049]
Incidentally, this self-flux method requires heating at a high temperature of about 1300 ° C or higher, unlike the two-stage firing method proposed by Subbarao (EC Subbarao, Phys. Solids. 23 (1962) 665.). However, a large amount of single-crystal thin film pieces free of impurities can be obtained. The self-flux method is a method using a substance constituting a part of the composition of a crystal to be grown as a flux (flux).
[0050]
As described above, according to the method for manufacturing a layered crystal structure oxide according to the present embodiment, substantially all of the flux is evaporated by maintaining the raw material M to which the flux is added at a temperature equal to or higher than the melting point for a predetermined time. Therefore, the grown layered crystal structure oxide can be directly taken out. Therefore, a post-process for taking out the layered crystal structure oxide is not necessary, and the layered crystal structure oxide can be prevented from being eroded by etching with an acid or the like. Furthermore, since the melting point increases as the flux decreases, the time required for the slow cooling step can be shortened and the layered crystal structure oxide can be produced efficiently.
[0051]
【Example】
Further, specific embodiments of the present invention will be described in detail with reference to the drawings. In the following examples, the stoichiometric composition formula is Bi.₂Sr₁Ta₂O₉The layered crystal structure oxide will be specifically described.
[0052]
(First embodiment)
Here, the fact that the layered crystal structure oxide can be produced by the production method according to the present embodiment and the conditions thereof will be specifically described.
[0053]
First, bismuth oxide, strontium carbonate, and tantalum oxide (all of which are special grade reagents; manufactured by High Purity Chemical Research Laboratories) were prepared as raw materials, and the molar ratio of bismuth oxide 79.0, strontium carbonate 10.5, and tantalum oxide 10.5. Mixed. That is, bismuth oxide was used as the flux here.
[0054]
Next, the double crucible 20 shown in FIG. 2 was prepared, and the raw material M mixed in the inner platinum crucible 21 was put, and then put in the outer alumina crucible 22 and covered with the lid 23. Subsequently, the double crucible 20 was inserted into the vertical siliconit furnace 30 shown in FIG. 3, and heat treatment was performed under the temperature conditions shown in FIG. That is, heating to 1400 ° C. at a heating rate of 300 ° C./hour (heating step), maintaining at 1400 ° C. for 1 hour or longer (constant temperature step), and cooling to 1150 ° C. at a cooling rate of 2 ° C./hour. (Slow cooling step) After that, the furnace was cooled to room temperature. At this time, the maintenance time in the constant temperature step was changed to 1 hour, 2 hours and 3 hours, respectively.
[0055]
As a result, the flux was completely evaporated in any of the maintenance times, and only the grown crystal was contained in the platinum crucible 21. The obtained crystal was yellow when the maintenance time was 1 hour, colorless and transparent when the maintenance time was 2 hours, and white when the maintenance time was 3 hours. The maximum size of each crystal was about 3 mm × 4 mm × 5 μm.
[0056]
These were each subjected to identification analysis by X-ray diffraction. The results are shown in FIGS. FIG. 6 shows an X-ray diffraction pattern (XRDP; X-ray Diffraction Patterns) of a crystal with a maintenance time of 2 hours, together with a reference pattern for comparing the XRDP. FIG. 7 shows the XRDP of each crystal with the maintenance time changed. The reference pattern was determined by the lattice constants (a = 2.49839 nm, b = 0.55344 nm, c = 0.55306 nm; AD Rae, JG Thompson and RL Withers, Acta. Cryst., B48 (1992) 418.) was obtained by Rietveld simulation, but “Fmmm” having higher symmetry was used for the space group.
[0057]
As shown in FIG. 6, it was found that the crystal obtained with a maintenance time of 2 hours was a layered crystal structure oxide composed of bismuth, strontium, tantalum and oxygen. On the other hand, as shown in FIG. 7A, the crystals obtained with the maintenance time of 1 hour are α-Bi as the impurity phase.₂O_Three(JCPDS No. 27-53) and, as shown in FIG. 7 (c), the crystals obtained with a maintenance time of 3 hours are SrTa as the impurity phase._FourO₁₁(JCPDS No.16-708) and Sr₂Ta₂O₇(JCPDS No. 30-1304) was found to be included.
[0058]
Furthermore, it can be seen from the XRDP shown in FIG. 6 that the crystal obtained with a maintenance time of 2 hours has a strong a-axis orientation due to the flake shape. The diffraction peaks (022) and (1800), the a-axis and c-axis lattice constants were found to be 2.498 nm based on the model that this crystal is a tetragonal crystal of b = c, and the c-axis lattice constant was 2.498 nm. Was 0.5528 nm. Since this crystal has an a-axis orientation, (00l) diffraction and (0k0) diffraction are extremely reduced, and there is an overlap with other peaks at a high angle. It is very difficult to obtain the axial lattice constant together. Rae et al. Show that this layered crystal structure oxide is a space group A2.₁am (Cmc2 in the notation in this embodiment₁(AD Rae, JG Thompson and RL Withers, Acta. Cryst., B48 (1992) 418.), but only from the X-ray diffraction results, the c-axis and b-axis It was not possible to determine whether the lattice constants were the same.
[0059]
In addition, for crystals obtained with a maintenance time of 2 hours, (1) microstructure observation with a scanning electron microscope (SEM), (2) surface morphology observation with an AFM (Atomic Force Microscope), (3) Laue Observation of spots, (4) Chemical analysis by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscope) and EPMA (Electron Probe Microanalysis), (5) Thermal analysis, and (6) Ferroelectric hysteresis observation. The results are shown below.
[0060]
(1) Results of microstructure observation by SEM
When the surface of the thin piece was observed by SEM, a smooth surface was observed. This is considered to be a-plane due to strong cleavage.
[0061]
(2) Results of surface morphology observation by AFM
When observed with a scanning range of 1 mm × 1 mm by AFM, a smooth surface was observed, and its RMS value (Root Mean Square) was about 0.4 nm. The step on the outermost surface has a height of 1.2 to 1.3 nm, which corresponds to half of the lattice constant of the a axis (about 2.5 nm) obtained from the result of X-ray diffraction. I understood. That is, this step is thought to be due to strong a-plane cleavage or anisotropic growth.
[0062]
(3) Observation results of Laue spots
FIG. 8 shows a photomicrograph of observed Laue spots. Thus, Laue spots were observed, and it was found that the obtained crystal was a single crystal.
[0063]
(4) Results of chemical composition analysis by ICP-AES and EPMA
Table 1 shows the analysis results of ICP-AES and EPMA. Thus, the composition ratio of bismuth, strontium, and tantalum was 1.92: 1.10: 2.00 according to ICP-AES and 1.87: 1.28: 2.00 according to EPMA. . That is, the obtained single crystal is considered to be close to the stoichiometric composition, although slight non-stoichiometry due to the metal can be considered. Note that EPMA is an analysis by wavelength dispersion (WDS; Wavelength Dispersive X-ray Spectroscopy).
[0064]
[Table 1]
ICP-AES; Bi: Sr: Ta = 1.92: 1.10: 2.00
EPMA (WDS); Bi: Sr: Ta = 1.87: 1.28: 2.00
[0065]
(5) Thermal analysis results
FIG. 9 shows the result of thermal analysis. Thus, the weight loss (TG) was very small at 0.3% or less over a wide temperature range. That is, it was found that once the crystals were synthesized at a high temperature of about 1400 ° C., evaporation of the bismuth compound and oxygen deficiency hardly occurred in the temperature range up to 1200 ° C.
[0066]
(5) Observation results of hysteresis
A platinum (Pt) electrode was deposited on one side of the crystal piece, and ferroelectric hysteresis was observed at 200 ° C. The measurement was performed at a high temperature because this layered crystal structure oxide has a very high coercive electric field, so it is difficult to observe at room temperature. FIG. 10 shows the result. Thus, ferroelectric hysteresis was observed, and it was found that this layered crystal structure oxide exhibits ferroelectricity.
[0067]
The raw material is heated to 1350 ° C. or higher and 1450 ° C. or lower and maintained for longer than 1 hour and shorter than 3 hours, and then gradually cooled to 800 ° C. or higher and 1300 ° C. or lower at a slow cooling rate of 1 ° C./hour to 5 ° C./hour. Also when the layered crystal structure oxide is grown by slow cooling, the same result as in the case where the maintenance time is 2 hours in the first embodiment can be obtained.
[0068]
In this example, the same raw material as in the previous example was prepared, and this raw material was mixed in the same mixing ratio as in the previous example. Next, heat treatment was performed using the double crucible 20 (see FIG. 2) and the vertical siliconite furnace 30 (see FIG. 3) under the temperature conditions shown in FIG. That is, it is heated to 1400 ° C. at a heating rate of 300 ° C./hour (heating step), maintained at 1400 ° C. for 10 hours or more (constant temperature step), and then gradually cooled to 1150 ° C. at a cooling rate of 5 ° C./hour. (Slow cooling step) After that, the furnace was cooled to room temperature. At this time, the maintenance time in the constant temperature step was changed to 10 hours, 30 hours and 40 hours, respectively.
[0069]
As a result, the flux was completely evaporated in any of the maintenance times, and only the grown crystal was contained in the platinum crucible 21. The obtained crystal was yellow when the maintenance time was 10 hours, colorless and transparent when the maintenance time was 30 hours, and white when the maintenance time was 40 hours. The maximum size of each crystal was about 3 mm × 4 mm × 5 μm.
[0070]
These were each subjected to identification analysis by X-ray diffraction. The result is shown in FIG. Thus, it was found that the crystal obtained with a maintenance time of 30 hours was a layered crystal structure oxide composed of bismuth, strontium, and tantalum oxygen. On the other hand, the crystals obtained with the maintenance time of 10 hours are δ-Bi as the impurity phase.₂O_Three(JCPDS No. 27-52), and the crystal obtained with a maintenance time of 40 hours is SrTa as the impurity phase._FourO₁₁(JCPDS No.16-708) and Sr₂Ta₂O₇(JCPDS No. 30-1304) was found to be included.
[0071]
The raw material is heated to 1350 ° C. or higher and 1450 ° C. or lower and maintained for a time longer than 10 hours and shorter than 40 hours, and then at a slow cooling rate of 5 ° C./hour to 20 ° C./hour to 800 ° C. to 1300 ° C. Even when the layered crystal structure oxide is grown by slow cooling, the same result as in the case where the maintenance time is 30 hours in the second embodiment can be obtained.
[0072]
In this example, the same raw material as in the previous example was prepared, and this raw material was mixed at the same blending ratio as in the previous example. Next, heat treatment was performed using the double crucible 20 (see FIG. 2) and the vertical siliconite furnace 30 (see FIG. 3) under the temperature conditions shown in FIG. That is, after heating to 1345 ° C. at a heating rate of 300 ° C./hour (heating step) and maintaining at 1345 ° C. for 30 hours or more (constant temperature step), it is gradually cooled to 1150 ° C. at a cooling rate of 2 ° C./hour. (Slow cooling step) After that, the furnace was cooled to room temperature. At this time, the maintenance time in the constant temperature step was changed to 30 hours, 40 hours, 45 hours and 50 hours, respectively.
[0073]
As a result, the flux having any maintenance time was completely evaporated, and only the grown crystal was contained in the platinum crucible 21. The obtained crystals were yellow for those having a maintenance time of 30 hours and 40 hours, colorless and transparent for those having a maintenance time of 45 hours, and white for those having a maintenance time of 50 hours. The maximum size of each crystal was about 3 mm × 4 mm × 5 μm.
[0074]
These were each subjected to identification analysis by X-ray diffraction. The result is shown in FIG. Thus, it was found that the crystal obtained with a maintenance time of 45 hours was a layered crystal structure oxide composed of bismuth, strontium, and tantalum oxygen. On the other hand, the crystals obtained with a maintenance time of 30 hours have α-Bi as the impurity phase.₂O_Three(JCPDS No. 27-53), and the tendency is the same for the crystals obtained with a maintenance time of 40 hours. Most of the crystals obtained with a maintenance time of 50 hours are SrTa in the impurity phase._FourO₁₁(JCPDS No. 16-708).
[0075]
The raw material is heated to 1330 ° C. or higher and 1370 ° C. or lower and maintained for a time longer than 40 hours and shorter than 50 hours, and then from 800 ° C. to 1300 ° C. at a slow cooling rate of 1 ° C./hour to 5 ° C./hour. Even when the layered crystal structure oxide is grown by slow cooling, the same result as in the case where the maintenance time is 45 hours in the third embodiment can be obtained.
[0076]
From the above results, the raw material added with bismuth oxide as a flux is heated to a predetermined temperature above the melting point, maintained at the predetermined temperature for a certain period of time, and then gradually cooled at a predetermined rate to evaporate substantially all of the flux. It was found that a single crystal of a layered crystal structure oxide can be produced. At that time, if the maintenance time in the constant temperature process is short, α-Bi₂O_Three(JCPDS No.27-53) or δ-Bi₂O_ThreeIf the impurity phase of (JCPDS No.27-52) is generated and the maintenance time is long, SrTa_FourO₁₁(JCPDS No.16-708) and Sr₂Ta₂O₇It was found that an impurity phase of (JCPDS No. 30-1304) was generated.
[0077]
(Second embodiment)
In this example, the crystal chemical structure of the layered crystal structure oxide will be specifically described.
[0078]
First, bismuth oxide, strontium carbonate, and tantalum oxide (all of which are special grade reagents; manufactured by High Purity Chemical Research Laboratories) were prepared as raw materials, and the molar ratio of bismuth oxide 79.0, strontium carbonate 10.5, and tantalum oxide 10.5. Mixed. That is, bismuth oxide was used as the flux here.
[0079]
Next, the double crucible 20 shown in FIG. 2 was prepared, and the raw material M mixed in the inner platinum crucible 21 was put, and then put in the outer alumina crucible 22 and covered with the lid 23. Subsequently, this double crucible 20 was inserted into the vertical siliconit furnace 30 shown in FIG. 3, heated to 1345 ° C. at a temperature rising rate of 300 ° C./hour (heating step), and maintained at 1345 ° C. for 20 hours. Thereafter (constant temperature process), the film was gradually cooled to 1150 ° C. at a slow cooling rate of 2 ° C./hour (slow cooling process), and then cooled to room temperature to produce a layered crystal structure oxide.
[0080]
Thereby, the flux was completely evaporated, and a lamellar crystal structure oxide crystal 1 having a size of about 0.7 mm × 0.2 mm × 0.01 mm was obtained.
[0081]
Moreover, the raw material was prepared similarly to the case of the crystal | crystallization 1, and it put into the double crucible 20 shown in FIG. 2, and inserted in the vertical siliconite filter 30 shown in FIG. Heat to 1400 ° C. at a heating rate of 300 ° C./hour (heating step), maintain at 1400 ° C. for 2 hours (constant temperature step), and gradually cool to 1200 ° C. at a cooling rate of 5 ° C./hour (slow cooling) Step), and then furnace-cooled to room temperature to produce a layered crystal structure oxide.
[0082]
That is, the heating temperature was slightly higher than that of the crystal 1, the maintenance time in the low-temperature process was short, and the slow cooling rate in the slow cooling process was relatively high. Thus, unlike the crystal 1, the flux is not completely evaporated, and some α-Bi is contained in the platinum crucible 21.₂O_ThreeHowever, a lamellar crystal structure oxide crystal 2 having a size of about 0.7 mm × 0.2 mm × 0.04 mm was obtained. The crystal 1 and crystal 2 thus obtained were analyzed as follows.
[0083]
First, 4-axis X-ray diffraction was performed. The apparatus used was a Rigku AFC5R diffrastometer, and the X-rays used were Mo-Kα rays monochromatized with graphite. As a result, from preliminary measurements, Bravais lattices are each C-centered
orthohombic, and the lattice constants are as shown below.

[0084]
Further, according to the disappearance rule of diffraction spots, the crystal space group of crystal 1 and crystal 2 is squeezed to either Pnnm or Pnn2 under the assumption that twins do not exist, and under the assumption that twins exist. Cmc2₁, Cmcm, or C2 cm. Here, analysis results by Newham and Rae et al. (RE Newnham, RW Wolfe, RS Horsey, FA Diaz-Colon and MI Kay, Mater. Res. Bull., 8 (1973) 1183.; AD Rae, JG Thompson and RL Withers , Acta. Cryst., B48 (1992) 418.), and the diffraction intensity distribution was statistically analyzed for each of these. Finally, twins existed in both crystals 1 and 2, and the crystal space group Is Cmc2₁It was determined that
[0085]
This space group is a standard setting published in “International Tables for Crystallography”. A2 is a setting that uses the c-axis as a major axis that is commonly used.₁am, which is consistent with the analysis results of Newham and Rae et al.
[0086]
Further, the difference in the lattice constants of the b-axis and the c-axis was not observed from the refined results in the crystal 1 and the crystal 2 in the refinement results, but it was concluded that they were not equivalent due to the spatial symmetry. Can be attached. In order to support this, each of the crystals 1 and 2 was observed with a polarizing microscope under a crossed Nicol state. The observation surface is a-plane. As a result, periodic bright and dark inversions were observed in both crystals 1 and 2, and it was clearly confirmed that the lattice constants of the b-axis and c-axis were not equal.
[0087]
Next, refinement of the crystal structure was performed by least square refinement by the Patterson method using analysis software “SAPI91” (Fan Hai-Fu (1991). Structure Analysis Programs With Intelligent Control, Rigaku Corporation, Tokyo, Japan.) went. Table 2 shows the atomic coordinates of crystal 1 (based on an initial analysis that assumes twinning and does not assume partial substitution of elements). As a result, it was found from the basic crystal structure (space group; I4 / mmm) shown in FIG. 1 that the atomic site was displaced, and the crystal had spontaneous polarization in the c-axis direction (space group; Cmc2₁). The reliability factor (R-factor) by this refinement was 3.7%.
[0088]
[Table 2]

[0089]
Further, in order to converge this reliability factor, the crystal structure of the crystal 1 was refined by establishing a model in which bismuth in the fluorite layer 11 and strontium in the perovskite layer 12 were partially mutually substituted. Table 3 shows the results. In Table 3, Bi (2) is bismuth present in the strontium site Sr (2), and Sr (1) is strontium present in the bismuth site Bi (1). From this result, it was confirmed that the crystal 1 had a mutual substitution of bismuth and strontium of 5% at the bismuth site and about 2% at the strontium site. The reliability factor also improved slightly, at 3.5%.
[0090]
[Table 3]

[0091]
In a partially inter-substituted model, the number of refinement parameters is large, so it can be said that the reliability factor is improved mathematically, but there is no problem in the scattering ability of X-rays for heavy elements, Further, since the absolute value of the reliability factor is statistically enough to suggest that this model is correct, it is considered that in this crystal 1, bismuth and strontium are partially substituted.
[0092]
Based on this result, the composition ratio of bismuth, strontium, and tantalum of crystal 1 was determined, and the result was as follows.
Crystal 1 Bi: Sr: Ta = 1.92: 1.09: 2
[0093]
In addition, with respect to the crystal 2, the difference between the simulation data and the actually measured diffraction data was statistically too large in the model that did not assume partial substitution of elements, and the reliability factor did not converge. Therefore, in order to converge the reliability factor, the crystal structure was refined by establishing a model in which bismuth in the fluorite layer 11 and strontium in the perovskite layer 12 were partially substituted, as in the case of the crystal 1. . Table 4 shows the results. Also in Table 3, Bi (2) is bismuth existing in the strontium site Sr (2), and Sr (1) is strontium existing in the bismuth site Bi (1). From this result, in Crystal 2, significant mutual substitution between bismuth and strontium of 42% at the bismuth site and 26% at the strontium site was observed. The reliability factor was 3.2%. Since the reliability factor has converged to a reliable level, it is considered that in crystal 2, bismuth and strontium are partially substituted.
[0094]
[Table 4]

[0095]
Based on this result, the composition ratio of bismuth, strontium, and tantalum of the crystal 2 was obtained, and the result was as follows.
Crystal 2 Bi: Sr: Ta = 1.43: 1.58: 2
[0096]
From these results, it is possible to consider the crystal chemical change of the entire crystal structure that occurs with the partial substitution of bismuth and strontium as follows. That is, Bi³⁺8b-site where there is Sr²⁺Since the multiplicity is twice as large as 4a-site in which Bi is present, if mutual substitution occurs at these sites, Bi³⁺Decreases and Sr²⁺Shows a tendency to increase. As a result, the positive charge in the crystal is reduced, and oxygen defects are required to maintain charge neutrality.
[0097]
As can be seen from FIG. 1, this layered crystal structure oxide has five different oxygen sites, oxygen O (1) in the SrO plane, oxygen O (positioned at the boundary between the fluorite layer 11 and the perovskite layer 12 ( 2) Oxygen in the fluorite layer 11 (3), TaO₂In-plane oxygen O (4) and O (5) can be described respectively. Note that O (4) and O (5) are very similar in chemical environment, but the symmetry reduction (F4 / mmm to Cmc2) due to lattice distortion.₁). Here, looking at Table 4, only the filling rate of oxygen O (3) in the fluorite layer 11 among these oxygens is extremely small, and the oxygen at this O (3) site is selectively reduced. It turns out that it is easy to lose. This is sufficiently considered from the large nonstoichiometry of the fluorite phase.
[0098]
What is more characteristic is that the O (2) site is separated into two sites with O (2) and O (2 ') filling ratios of approximately 50%. However, in reality, it is unlikely that the closest O (2) site and O (2 ′) site are occupied at the same time because clone repulsion increases, and the nearest O (2) site is caused by disorder. (2) It is considered that either the site or the O (2 ′) site is occupied.
[0099]
Although such a defect structure seems to be universal to some extent, it is considered that this defect structure is basically caused by the mutual partial substitution of bismuth and strontium recognized in this example. It is considered that the defect structure changes variously, such as unilateral partial substitution with bismuth and substitution between different sites.
[0100]
FIG. 15 shows the crystal structure of the crystal 1 faithfully reproduced including the lattice distortion based on the above results. The crystal structure of crystal 2 reproduced in the same manner is shown in FIG. 15 and 16, one unit cell is shown in the a-axis direction, and two unit cells are shown in the b-axis and c-axis directions. By the way, to avoid complications TaO₆This structure is shown as an octahedron, but tantalum and oxygen are shown as atoms in the part where a perfect octahedron is not formed in the boundary region.
[0101]
FIG. 17 shows the crystal structure of FIG. 15 (that is, crystal 1) viewed from the b-axis direction, and FIG. 18 shows the crystal structure viewed from the c-axis direction. Similarly, FIG. 19 shows a crystal structure of FIG. 16 (that is, crystal 2) viewed from the b-axis direction, and FIG. 20 shows a crystal structure viewed from the c-axis direction. From FIG. 17 and FIG. 18, in the crystal 1, the oxygen plane of the fluorite layer 11 is almost flat.₆In the octahedron₂It can be seen that the inclination of the surface occurs around the c-axis that is the polarization axis (that is, the rotation axis is the c-axis). Further, from FIGS. 19 and 20, it can be seen that in the crystal 2, the oxygen plane of the fluorite layer 11 is bent. In the crystal 2, as described above, there are considerable defects in the oxygen sites, and the apex oxygen sites at the boundary with the fluorite layer 11 are separated as described above (here, the interatomic distance from tantalum is The octahedron is represented with the smaller O (2 ′) as the vertex).
[0102]
Furthermore, from FIG. 17 to FIG. 20, in the crystals 1 and 2, not only the polarization of the perovskite layer 12, but also the relative displacement of bismuth and oxygen in the fluorite layer 11 is never small, and they contribute to the polarization. Conceivable. A cation (bismuth or strontium substituted with bismuth site) and an anion (oxygen O (3)) in the fluorite layer 11 based on a matrix structure (F4 / mmm) that is actually free from lattice distortion. When the polarization axis component of the relative displacement is calculated, the unit cell length of the polarization axis (c axis) (C₀ Approx. 5.57% for crystal 1 and about 5.47% for crystal 2. In general, it is considered that the perovskite layer 12 is mainly responsible for the polarization, and there is no example in which such a large polarization is confirmed to occur in the fluorite layer 11.
[0103]
By the way, analysis results by Newnham and Rae et al. (RE Newnham, RW Wolfe, RS Horsey, FA Diaz-Colon and MI Kay, Mater. Res. Bull., 8 (1973) 1183.; AD Rae, JG Thompson and RL Withers, Acta. Cryst., B48 (1992) 418.), the polarization axis components of the relative displacement between the cation and the anion in the fluorite layer are 2.80% and 1.69%, respectively.
[0104]
Here, the unit cell length of the polarization axis is defined in the same manner as the normal fluorite phase, and is about 5.5 mm. However, for example, when a long-period structure exists on the polarization axis or the bc plane and ca plane including the polarization axis, the lattice constant increases, and the calculated value of the polarization axis component of this relative displacement is apparently reduced. Therefore, in such a case, the polarization axis component of relative displacement is estimated with respect to the unit cell length corresponding to a normal fluorite phase.
[0105]
Further, FIG. 21 shows the crystal structure of the fluorite layer 11 extracted from the crystal 1 and the apex oxygen adjacent thereto extracted from the a-axis direction. Similarly, the fluorite layer 11 of the crystal 2 is adjacent to this. FIG. 22 shows a crystal structure obtained by extracting the apex oxygen to be extracted and viewed from the a-axis direction. Further, TaO of the perovskite layer 12 of the crystal 1₆FIG. 23 shows a crystal structure obtained by extracting the octahedron and strontium adjacent to the octahedron and viewing from the a axis. Similarly, TaO of the perovskite layer 12 in the crystal 2 is shown.₆FIG. 24 shows a crystal structure obtained by extracting the octahedron and strontium adjacent to the octahedron and viewing from the a-axis.
[0106]
From FIGS. 21 and 22, it can be seen from FIG. 21 that the bonding of oxygen O (3) in the fluorite layer 11 in the crystal 1 is almost flat and less distorted. It can be seen that it is distorted greatly. It can also be seen that the disorder of apex oxygen recognized in the crystal 2 is not the polarization axis direction (c-axis direction) or the direction perpendicular to it (b-axis direction), but close to the (b + c) direction.
[0107]
Further, from FIGS. 23 and 24, in the crystal 1, as described above, TaO₂It can be seen that the inclination of the surface occurs around the c-axis, which is the polarization axis, but the apex oxygen also has a component of b-axis rotation. On the other hand, in crystal 2, TaO combined with disorder of apex oxygen₂It can be seen that the bond of is distorted in the bc plane.
[0108]
As described above, in this layered crystal structure oxide, bismuth and strontium are partially substituted, or oxygen constituting the fluorite layer 11 is selectively lost or located at the boundary between the fluorite layer 11 and the perovskite layer 12. It has been found that apex oxygen is disordered, or that the cation and the anion of the fluorite layer 11 are displaced relative to each other in the polarization axis direction, thereby exhibiting a specific paraelectric property or ferroelectricity.
[0109]
Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to these embodiments and examples and can be variously modified. For example, in the above example, the stoichiometric composition formula is Bi.₂Sr₁Ta₂O₉Although the present invention has been described with reference to the layered crystal structure oxide, the present invention can obtain the same results for other layered crystal structure oxides of the so-called Aurivirius crystal group represented by the chemical formula shown in Chemical Formula 1.
[0110]
Specifically, Bi in m is 2 in chemical formula 1₂BiTiNbO₉And Bi₂BiTiTaO₉A layered crystal structure oxide represented by the chemical formula: Bi wherein m is 3_FourTi_ThreeO₁₂A layered crystal structure oxide represented by the chemical formula: Bi wherein m is 4₂Bi₂CaTi_FourO₁₅  , Bi₂Bi₂SrTi_FourO₁₅  , Bi₂Bi₂BaTi_FourO₁₅  , Bi₂Bi₂PbTi_FourO₁₅  , Bi₂Bi_2.5Na_0.5Ti_FourO₁₅And Bi₂Bi_2.5K_0.5Ti_FourO₁₅A layered crystal structure oxide represented by the chemical formula:₂Bi₂Sr₂Ti_FiveO₁₈  , Bi₂Bi₂Ba₂Ti_FiveO₁₈And Bi₂Bi₂Pb₂Ti_FiveO₁₈The present invention is similarly applied to a layered crystal structure oxide represented by the chemical formula:
[0111]
The present invention is similarly applied not only to the so-called Aurivirius crystal group but also to a layered crystal structure oxide known by the chemical formula shown in Chemical Formula 3 and known as a superconducting material.
[Chemical Formula 3]
Bi₂Sr₂Ca_n-1Cu_nO_{2n + 4}      N ≦ 4
[0112]
This is called a so-called Bi-based superconducting layered oxide, and its crystal structure is similar to the Auribilius crystal group. In particular, the fact that the layers are partitioned by Bi-O is very similar in terms of anisotropy.
[0113]
Furthermore, in the above embodiment and examples, substantially all of the flux is evaporated in the production of the layered crystal structure oxide. However, if the grown layered crystal structure oxide is evaporated to such an extent that it can be taken out, Effects similar to those of the embodiment and the experimental example can be obtained.
[0114]
In addition, in the above-described embodiments and examples, the raw material to which the flux is added in the production of the layered crystal structure oxide is heated to a predetermined temperature equal to or higher than the melting point and maintained at the predetermined temperature for a certain time, and then the natural cooling is performed. By slowly cooling at a slow speed, the temperature was maintained at a temperature above the melting point for a predetermined time to evaporate substantially all of the flux. However, the present invention maintains the temperature at a temperature above the melting point for a predetermined time by another method. May be evaporated substantially entirely.
[0115]
For example, the raw material to which the flux is added may be heated to a predetermined temperature equal to or higher than the melting point and maintained at the predetermined temperature for a certain period of time to maintain the temperature equal to or higher than the melting point for a predetermined time so that substantially all of the flux is evaporated. . Alternatively, the raw material to which the flux is added may be heated to a predetermined temperature equal to or higher than the melting point and then allowed to cool and held at a temperature equal to or higher than the melting point for a predetermined time to evaporate substantially all of the flux. Furthermore, after heating the raw material to which the flux has been added to a predetermined temperature above the melting point, it is gradually cooled at a slower rate than natural cooling and held at a temperature above the melting point for a predetermined time so that substantially all of the flux is evaporated. Also good.
[0119]
【The invention's effect】
BookAccording to the method for producing the layered crystal structure oxide of the invention, the flux is evaporated to such an extent that the layered crystal structure oxide can be taken out by holding the raw material to which the flux is added at a temperature equal to or higher than the melting point for a certain time. Therefore, the grown layered crystal structure oxide can be directly taken out. Therefore, there is an effect that a post-process for taking out the layered crystal structure oxide becomes unnecessary, and that the layered crystal structure oxide can be avoided from being eroded by etching with an acid or the like. Moreover, since melting | fusing point becomes high when flux becomes small, the time which manufacture requires can be shortened and there exists an effect that a layered crystal structure oxide can be manufactured efficiently.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a basic crystal structure of a layered crystal structure oxide according to an embodiment of the present invention.
FIG. 2 is a structural diagram showing a double crucible used when manufacturing the layered crystal structure oxide shown in FIG. 1;
3 is a configuration diagram showing a vertical siliconite furnace used in manufacturing the layered crystal structure oxide shown in FIG. 1. FIG.
4 is a view showing temperature conditions in heat treatment when manufacturing the layered crystal structure oxide shown in FIG. 1. FIG.
FIG. 5 is a diagram illustrating temperature conditions during heat treatment in the first embodiment of the present invention.
6 is an X-ray diffraction pattern diagram (a) showing the composition of one crystal obtained under the temperature condition shown in FIG. 5, and it is shown together with a Rietveld simulation pattern (b).
7 is an X-ray diffraction pattern diagram for explaining the difference in the composition of crystals obtained by the maintaining time of the constant temperature process under the temperature condition shown in FIG. 5. FIG.
8 is a photomicrograph showing Laue spots of the layered crystal structure oxide obtained under the temperature condition shown in FIG.
9 is a characteristic diagram showing the results of thermal analysis for the layered crystal structure oxide obtained under the temperature conditions shown in FIG. 5. FIG.
10 is a characteristic diagram showing ferroelectric hysteresis of a layered crystal structure oxide obtained under the temperature condition shown in FIG.
FIG. 11 is a diagram showing another temperature condition during the heat treatment in the first embodiment of the present invention.
12 is an X-ray diffraction pattern diagram for explaining a difference in the composition of crystals obtained by the maintaining time of the constant temperature process under the temperature condition shown in FIG. 11. FIG.
FIG. 13 is a diagram showing still another temperature condition during the heat treatment in the first embodiment of the present invention.
14 is an X-ray diffraction pattern diagram for explaining a difference in composition of crystals obtained depending on the maintaining time of the constant temperature process under the temperature condition shown in FIG.
FIG. 15 is a conceptual diagram showing the crystal structure of the layered crystal structure oxide (crystal 1) according to the second embodiment of the present invention including the lattice strain.
FIG. 16 is a conceptual diagram showing the crystal structure of the layered crystal structure oxide (crystal 2) according to the second embodiment of the present invention including the lattice strain.
17 is a conceptual diagram showing the crystal structure of the layered crystal structure oxide (crystal 1) shown in FIG.
18 is a conceptual diagram showing the crystal structure of the layered crystal structure oxide (crystal 1) shown in FIG.
FIG. 19 is a conceptual diagram illustrating the crystal structure of the layered crystal structure oxide (crystal 2) shown in FIG.
20 is a conceptual diagram showing the crystal structure of the layered crystal structure oxide (crystal 2) shown in FIG.
21 is a conceptual diagram showing a crystal structure including a lattice strain extracted from a fluorite layer and apex oxygen adjacent to the fluorite layer in the layered crystal structure oxide (crystal 1) shown in FIG. 15 and viewed from the a-axis direction. It is.
22 is a conceptual diagram showing a crystal structure including a lattice strain extracted from a fluorite layer and apex oxygen adjacent to the fluorite layer in the layered crystal structure oxide (crystal 2) shown in FIG. 16 and viewed from the a-axis direction. It is.
FIG. 23 shows TaO of the perovskite layer in the layered crystal structure oxide (crystal 1) shown in FIG.₆FIG. 6 is a conceptual diagram illustrating a crystal structure including an octahedron and strontium adjacent thereto extracted from the a-axis direction and including lattice distortion.
24 is a TaO of a perovskite layer in the layered crystal structure oxide (crystal 2) shown in FIG.₆FIG. 6 is a conceptual diagram illustrating a crystal structure including an octahedron and strontium adjacent thereto extracted from the a-axis direction and including lattice distortion.
FIG. 25 is a conceptual diagram showing a crystal structure of a fluorite phase.
FIG. 26 is a conceptual diagram showing a crystal structure of a perovskite phase.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Fluorite layer, 12 ... Perovskite layer, 20 ... Double crucible, 21 ... Platinum crucible, 22 ... Alumina crucible, 23 ... Lid, 30 ... Vertical siliconite furnace, 31 ... Core tube, 32 ... Mounting stand, 33 ... Heater, 34 ... heat insulation, 35 ... lid, M ... raw material

Claims (22)

A method for producing a layered crystal structure oxide using a self-flux method to produce a layered crystal structure oxide,
After heating the raw material to which the flux has been added to a temperature equal to or higher than the melting point, the flux is evaporated to such an extent that the layered crystal structure oxide can be taken out by maintaining the temperature at a temperature equal to or higher than the melting point for a certain period of time. A method for producing a layered crystal structure oxide, characterized by comprising: After heating the material obtained by adding a flux to a constant temperature above the melting point, by a predetermined time maintained at that temperature, the layered crystal structure oxide of claim 1, wherein for a certain holding time to a temperature above the melting point Manufacturing method. After heating the material obtained by adding a flux to a constant temperature above the melting point, the production method according to claim 1 layered crystal structure oxide, wherein the retaining certain time to a temperature higher than the melting point while allowed to cool. After heating the material obtained by adding a flux to a certain temperature above the melting point, the layered crystal structure according to claim 1, wherein the retaining certain time at a temperature above the melting point while slowly cooling at a slower rate than natural cooling Production method of oxide. After heating the raw material to which the flux has been added to a certain temperature above the melting point, the material is maintained at that temperature for a certain period of time, and then gradually cooled at a slower rate than natural cooling and maintained at a temperature above the melting point for a certain period of time. The method for producing a layered crystal structure oxide according to claim 1 . The process according to claim 1 layered crystal structure oxide, wherein the producing a layered crystal structure oxide including at least bismuth bismuth oxide as a flux. 3. The method for producing a layered crystal structure oxide according to claim 2 , wherein a layered crystal structure oxide containing at least bismuth is produced using bismuth oxide as a flux. 4. The method for producing a layered crystal structure oxide according to claim 3 , wherein a layered crystal structure oxide containing at least bismuth is produced using bismuth oxide as a flux. 5. The method for producing a layered crystal structure oxide according to claim 4 , wherein a layered crystal structure oxide containing at least bismuth is produced using bismuth oxide as a flux. 6. The method for producing a layered crystal structure oxide according to claim 5 , wherein a layered crystal structure oxide containing at least bismuth is produced using bismuth oxide as a flux. A heating step of heating the raw material added with bismuth oxide as a flux to a temperature of 1330 ° C. or higher and 1450 ° C. or lower;
A constant temperature step of maintaining a constant temperature at the heated temperature in this heating step;
The method for producing a layered crystal structure oxide according to claim 10 , further comprising: a slow cooling step of gradually cooling to 800 ° C. or higher and 1300 ° C. or lower at a rate slower than natural cooling after the constant temperature step. The heating temperature of the heating step is 1350 ° C. or higher and 1450 ° C. or lower, the maintenance time of the constant temperature step is longer than 1 hour and shorter than 3 hours, and the slow cooling rate of the slow cooling step is 1 ° C./hour The method for producing a layered crystal structure oxide according to claim 11, wherein the temperature is 5 ° C./hour or less. The heating temperature of the heating step is 1350 ° C. or higher and 1450 ° C. or lower, the maintenance time of the constant temperature step is longer than 10 hours and shorter than 40 hours, and the slow cooling rate of the slow cooling step is 5 ° C./hour The method for producing a layered crystal structure oxide according to claim 11, wherein the temperature is 20 ° C./hour or less. The heating temperature of the heating step is set to 1330 ° C. or higher and 1370 ° C. or lower, the maintenance time of the constant temperature step is longer than 40 hours and shorter than 50 hours, and the slow cooling rate of the slow cooling step is 1 ° C./hour. The method for producing a layered crystal structure oxide according to claim 11, wherein the temperature is 5 ° C./hour or less. It consists of bismuth (Bi), a first element, a second element, and oxygen (O), and the first element is sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), and the second element is iron (Fe), titanium (Ti), vanadium (V), niobium (Nb), A layered crystal structure characterized by producing a layered crystal structure oxide which is at least one member selected from the group consisting of tantalum (Ta), tungsten (W) and copper (Cu) by the method according to claim 1. Production method of oxide. It consists of bismuth (Bi), a first element, a second element, and oxygen (O), and the first element is sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), and the second element is iron (Fe), titanium (Ti), vanadium (V), niobium (Nb), A layered crystal structure characterized in that a layered crystal structure oxide which is at least one member selected from the group consisting of tantalum (Ta), tungsten (W) and copper (Cu) is produced by the method according to claim 11. Production method of oxide. The chemical formula in which the first element is represented by Me and the second element is represented by R is Bi _2-a Me _{1 + b} R ₂ O _{9 + c} , and the first element is strontium (Sr), lead (Pb), barium ( Ba) and calcium (Ca), and the second element is at least one of the group consisting of niobium (Nb) and tantalum (Ta), and a, The layered crystal structure oxide according to claim 1 , wherein b and c each have a value in the range of 0≤a≤0.14, 0≤b≤0.35, -0.21≤c≤0.35. A method for producing a layered crystal structure oxide, characterized by comprising: The chemical formula in which the first element is represented by Me and the second element is represented by R is Bi _2-a Me _{1 + b} R ₂ O _{9 + c} , and the first element is strontium (Sr), lead (Pb), barium ( Ba) and calcium (Ca), and the second element is at least one of the group consisting of niobium (Nb) and tantalum (Ta), and a, The layered crystal structure oxide according to claim 11 , wherein b and c are values in the range of 0 ≦ a ≦ 0.14, 0 ≦ b ≦ 0.35, −0.21 ≦ c ≦ 0.35, respectively. A method for producing a layered crystal structure oxide, characterized by comprising: A method for producing a layered crystal structure oxide, comprising producing a layered crystal structure oxide exhibiting ferroelectricity by the method according to claim 1 . A method for producing a layered crystal structure oxide, comprising producing a layered crystal structure oxide exhibiting paraelectricity by the method according to claim 1 . _2. A layered crystal having a chemical formula of Bi ₂ Sr ₂ Cann _-1 Cu _n O _{2n + 4} , wherein n is an integer of 4 or less by the method according to claim 1. A method for producing a structural oxide. A layered crystal structure comprising: a layered crystal structure oxide having a chemical formula of Bi ₂ Sr ₂ Ca _n-1 Cu _n O _{2n + 4} and n in the chemical formula being an integer of 4 or less by the method according to claim 11. A method for producing a structural oxide.

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