JPH02196018A - Nd2cuo4-type oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain the useful oxide superconductor to be used at above the liquefied He temp. by specifying the crystal structure. CONSTITUTION:The oxides of the rare-earth elements other than Ce, Pr, Tb, Tm, Yb, Lu, and Se, the oxides, carbonate, peroxide, etc., of the alkali (alkaline earth) elements, namely Ba, Sr, Ca, and Na, and cupric oxide are mixed as the raw material in a desired cationic ratio. The mixture is heated in an oxidizing atmosphere, then crushed, mixed, and formed. The formed product is subjected to a reaction in the atmosphere of Ar, O2, etc., to eliminate the lattice defect of O2. Consequently, the oxide superconductor shown by the chemical formula (Ln is Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, and Y, M is Ba, Sr, Ca, and Na, 0.001<=x<=0.15) and having an Nd2CuO4-type crystal structure is obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an oxide superconductor, and relates to an oxide superconductor at a liquid helium temperature (4
The present invention relates to superconductors suitable for use at temperatures above K).

[Conventional technology]

All the crystal structures of cuprate superconductors discovered so far are classified as perovskite-related structures. Here, the crystal structure called perovskite includes: (1) a structure in which copper is the central element in a coordination polyhedron formed by copper and oxygen, and oxygen has an octahedral six-coordination configuration; (2) oxygen coordinated to copper; crystal structure in which one or two of the above are missing and the configuration of the coordination polyhedron formed by copper and oxygen is a pyramidal five-coordination or a planar four-coordination; This includes crystal structures such as In these perovskite-related structures, if the atomic coordinates of copper are (0, O, O), the atomic coordinates of oxygen are (1/2.
O, O), (0, 1/2.0), (0, O, z) (z≠
0).

On the other hand, in the Nd2Cub4 type structure, the atomic coordinates of the cation are approximately equivalent to La2Cub4, which is known as a typical perovskite-related structure. However, oxygen is located at partially different coordinates from the perovskite structure, and with respect to copper atoms, (1/2,0°2) and (0,1/2
．． z) (z=o and 2≠0). Therefore, the Nd2Cub4 type structure has copper and oxygen (2=0
), it is classified as a perovskite-related structure in that it has a four-coordinate planar configuration;
) is not located at any coordinate of the octahedral six-coordinated configuration, which is different from the perodisky 1-structure.

The mechanism by which superconductivity occurs in cuprate superconductors is not yet clear, but from the perspective of crystal structure, in any superconductor, Cu-0 is formed by two-dimensional bonding of clusters with 4-planar coordination. Planes exist in the crystal structure. Similar two-dimensional surfaces are also found in N d 2 Cu OI type structures.

For compounds with Nd2CuO4 type structure, the chemical formula is Ln,
Nd, Sm, Eu, Gd, etc., in which the element L n has a smaller ionic radius than L a when expressed as CaO2, are known J1
There are 4. Descriptions regarding these compounds are provided by R, Gran
de, Hk, Muller-Busehbaum a.
nd M, 5chilIeizer.

Z, anorg, allg, Chew,, 42
8 (1977) 120°Hk, Muller-Bu
schbaum and WJollschlaHer
.

Z, anorg, allg, Chew, 414
(1975) seen at 76°.

[Problem that the invention seeks to solve]

In any case, a superconductor with an Nd2Cub4 type structure has not been found. An object of the present invention is to provide a superconductor having an Nd2Cub4 type structure.

[Means for solving problems]

First, for the purpose of introducing holes into the Cu=02-dimensional plane, rare earth elements were added to alkaline earth elements (Ca, Sr
, Ba) or an alkali element (Na, K). Generally, if oxygen lattice defects are generated in these substitution products, holes are effectively introduced into the C+1-02 dimensional plane.

However, it is quite conceivable that substitution with an alkali (earth) element causes oxygen lattice defects to be formed and holes are not introduced. Therefore, in order to prevent the formation of oxygen lattice defects, consideration is given to synthesis under high pressure oxygen of about 100 atmospheres.

The amount of substitution is determined by changing the composition formula to (Ln, -xMx)2Cu044(L
n = rare earth element, M = alkaline earth or alkali element), and for alkaline earth elements and 3, 0
．． 02≦x≦0.15, 0.0 for alkali elements
The range of 1≦x≦0.08 is preferable. However, in order to obtain a high onset Tc of 20 K or more, the range of
For alkaline earth elements, 0.08≦x≦0.12,
The alkali element is preferably in the range of 0.04≦x≦0°06. Although no theoretical elucidation has yet been made regarding how the range of X is defined, (T
The same thing is known for ma, 5r)2Cub4.

The compounds are easily synthesized by in-phase reactions using raw powder samples as starting materials. For example, oxides for rare earth elements, oxides and carbonates for alkali and alkaline earth elements.

For copper, cupric oxide is used as a starting material. Mix each raw material powder according to the desired cation ratio, and the reaction will be 1 in an oxidizing atmosphere such as air or oxygen.
Heat to a temperature that accelerates the reaction for - minutes and hold for several hours to further accelerate the reaction. The synthesized compound powder is crushed, mixed, and then molded into tablets. In order to complete the reaction, the reaction is continued for several hours in an atmosphere of Ar, oxygen, etc. Reaction temperature 2 Reaction time varies depending on starting materials 7 composition 9 reaction atmosphere, etc., but for the first stage reaction, 800-950°C,
A temperature range of 900-i-200<0>C is common for the second stage reaction. The reaction time is generally 6 hours or more and around 12 hours.

If oxygen lattice defects are formed as a result of the above synthesis, annealing under high pressure oxygen is required as described above. The crystal lattice is heated to a temperature at which oxygen is sufficiently diffused to eliminate oxygen lattice defects. In the case of the crystal structure of this patent, a typical temperature is at least 400° C. or higher, and in order to shorten the annealing time, it is desirable to anneal at around 600° C. for around 12 hours.

[Effect]

The rare earth elements U, Ce, Pr, and Tb are excluded because they can be tetravalent, and Tm, Y'b, Lu, and Sc are excluded because their ionic radii are too small to maintain a stable crystal structure. Among the alkali (earth) elements, Mg, Be, and Li are also excluded because their ionic radii are too small. In other words, (L
For the chemical formula expressed as n, M)2CuO4, Ln
=Nd, Pm.

Sm, Eu, Gd, Dy, Ho, Er, Y, and M
=Ba, Sr, Ca, and Na are considered as constituent elements. Although many combinations of these elements are possible, no restrictions are imposed on the combinations of constituent elements. Also, other rare earth elements.

Alkali (earth) elements may be included in the crystal as long as they do not significantly increase the lattice constant. Therefore, one compound may contain a plurality of rare earth elements and a plurality of alkali/alkaline earth elements.

〔Example〕

Examples of the present invention will be given below and will be explained in more detail.

First, cupric oxide (Cub), neodymium oxide (Nd, 03) and samarium oxide (Sm2) are used as rare earth element sources.
0. ), any one of europium oxide (Eu20.), and calcium carbonate (C) as an alkaline earth element source.
aCOs), any one of strontium carbonate (SrCO□), sodium peroxide (Na20) as an alkali element source, chemical formula % formula %1 M: alkaline earth element) or (Lno, ssMo, os)zcuoy (L n
: rare earth element, M: alkali element). Next, each mixed powder was mixed in air at 800 to 900°C for 1 hour.
After being calcined for 2 hours, it was cooled in the furnace. The calcined powder was crushed and mixed, then molded into tablets, and reacted in an Ar stream at 800° C. for 12 hours to obtain a single-phase compound. This was heated at 800° C. for 12 hours in an oxygen stream of 1 atm or 100 atm, and then cooled in a furnace.

From the results of the X-ray diffraction patterns, all of the synthesized phases had a Nd2CuO4 type structure. In addition, the average valence of copper was determined by coulometric titration, and the oxygen content in the compound was roughly estimated. The superconducting critical temperature and superconducting volume fraction were determined from the measurement of DC magnetic susceptibility.

The table shows composition 2 synthesis conditions and onset Tc (temperature at which the magnetic susceptibility begins to show a negative value) for all samples.

final Tc (the temperature at which the magnetic susceptibility begins to show a constant negative value), the volume fraction of the part in the sample that exhibits complete diamagnetic property,
are shown together. The final Tc is low, 12K.
The highest one was 45.

Although the onset Tc was as high as 53, the resistivity in the normal state showed a significant semiconductor change.

Systems not listed in the table, (Ln, M) = (E r.

S r), (Pm, S r), (Pm, Ca), (Gd
.

s rL (adt ea), (Dy+ Ca), (D
y.

S r), (Ho, S r), (Ho, Ca), (Er
.

As a result of experiments on Sr) and (Er, Ca), superconductors with a final Tc of 15 to 40 were obtained. Regarding different compositions, when X was in the range of 0.08 to 0.12 for alkaline earth substitution systems, Tc comparable to that of x=0.1 was obtained. For X larger and smaller X, Tc becomes smaller and x<0
．． 01 and x > 0, 2, no large diamagnetic signal was observed.

〔Effect of the invention〕

The present invention can provide a superconductor with an Nd2CuO4 type structure.

Claims (1)

[Claims] 1. An oxide superconductor characterized in that its crystal structure is an Nd_2CuO_4 type structure. 2. The chemical formula is (Ln_1_-_xMx)_2CuO_4
＿−＿δ(Ln=Nd, Pm, Sm, Eu, Gd, Dy
, Ho, Er, Y, M: M=Ba, Sr, Ca, Na)
An oxide superconductor according to claim 1, which is expressed as: 3. The oxide superconductor according to claim 1 or 2, wherein the range of x is 0.01≦x≦0.15.