DE1195284B - Process for the production of single crystals of the manganites of the rare earths - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

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COIf

German class: 12 m -17/00

C 27198IV a / 12 m
June 8, 1962
June 24, 1965

The present invention relates primarily to the production of single crystals of the manganites of the rare earths, which _{correspond to the general formula MnO 3} T, in which T is a cation of the rare earths.

It is known that the elements from the group of rare earths depending on the number of electrons form two isomorphic rows of manganites in their N-shell.

If the number of extra 4f electrons of these elements is less than or equal to 9 like with lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium and dysprosium, the manganites obtained have a crystal structure that is related to that of the perovskites; if however, the number of additional 4f electrons is greater than 9 as in holmium, erbium, thulium, Ytterbium and Lutecium (Cassiopeium), the manganites obtained have a new crystal structure of the hexagonal system, which is not centrosymmetric and due to anomalous mutual assignment the manganese ions and the rare earth ions is characterized.

It is also known that the manganites obtained by conventional production methods (decomposition of nitrates of manganese and rare earths and combination of the oxides obtained in this way at high temperature) contain ^{manganese in the form of Mn 3+} and Mn ^4+. These compounds are semiconductors with low resistivity and therefore not suitable for use as dielectrics. Finally, it is known that the non-centrosymmetrical hexagonal crystals can have piezoelectric and ferroelectric properties which can be used well by cutting and metallizing parallel to their bases.

The present invention relates to a method for producing single crystals of manganites Rare earths, which are developed in parallel with the bases, have a high specific resistance and for have interesting piezoelectric and ferroelectric properties in practice.

The process according to the invention for the production of single crystals of manganites rare earths by crystallization from a melt of the components in a solvent is characterized in that manganese oxide (Mn ₂ O ₃ ) and the oxide of a trivalent rare earth at a temperature above 1000 ° C in a - based on the manganese oxide - about five times the molar amount of the method used as a solvent for growing single crystals of orthochromites of rare earths for the production of single crystals
Rare earth manganites

Applicant:

Commissariat a Pfinergie Atomique, Paris

Representative:

Dipl.-Ing. R. Beetz and Dipl.-Ing. K. Lamprecht, Patent Attorneys, Munich 22, Steinsdorfstr. 10

Named as inventor:

Erwin Felix Bertraut, Grenoble, Isere;

Francis Forrat Marcoussis, Seine-et-Oise

(France)

Claimed priority:

France of June 12, 1961 (864 585)

Known bismuth oxide (Bi ₂ O ₃ ) dissolves, the melt is initially supersaturated by slow evaporation of the bismuth oxide until the first manganite crystals appear in it, then in a known manner with a cooling rate of about 10 ° C / h either immediately to ambient temperature cools or only after gradual complete evaporation of the bismuth oxide cools down and optionally separates the single crystals from the solidified melt in a known manner.

The molten bismuth oxide, which is used as a solvent, has the advantages that it dissolves the oxides very well and gives crystals in which all of the manganese in the manganite state is in the form of Mn ³⁺ .

A further advantage of this production process is to be seen in the fact that crystals can be placed in plates or Platelet shape that are developed parallel to their bases wins. According to the new process obtained single crystals have very interesting ferroelectric properties that little can be influenced by the choice of the rare earth cation:

The ferroelectricity occurs only in the direction perpendicular to the plane of the plate-shaped single crystal;
it is stable between -100 and +100 ⁰ C,

509 597/340

which makes the use of the single crystals possible at ambient temperature;
the saturation polarization is at these temperatures in the vicinity of 3 μ C / m ^2, the coercive field is about 20 kV / cm (the measurements were performed on samples from some 1O ^-1 mm thickness performed).

A first variant of this process is that at a temperature above 1000 ° C a mixture of bismuth oxide, manganese oxide and the oxide of a rare earth melts, then the solution obtained by slowly evaporating the solvent for the oxides Bismuth oxide is oversaturated until the first crystals appear. In this state one leads slow cooling to ambient temperature to allow the crystals to grow and to avoid internal mechanical stresses.

A second variant of the method according to the invention consists in the fact that, as before, a mixture is made at a temperature above 1000.degree from bismuth oxide, manganese oxide and the oxide of a rare earth melts, then the The solution obtained is slowly evaporated until the bismuth oxide has completely disappeared; afterward one cools down to the ambient temperature.

The evaporation of the solvent (Bi ₂ O ₃ ) saturates the solution and crystallization begins. The decrease in supersaturation caused by the precipitation of the dissolved components in crystal form is compensated by the increase in supersaturation due to the continuous evaporation of the solvent in such a way that a supersaturation which is desirable for the formation of the single crystals is achieved.

The following are several examples of the implementation of the method according to the invention for the production of single crystals of the rare earth manganites described in more detail.

example 1

_{A homogeneous mixture of 15 mol Bi 2} O ₃ , 3 mol Mn ₂ O ₃ and 3 mol Tm ₂ O ₃ (thulium oxide) is melted in a platinum bar at 1350 ° C. The molten solution is supersaturated by slow evaporation at a temperature of 1400 ° C. until the first crystals appear, corresponding to the first variant of the method according to the invention. Then, the temperature by 4 ° C / up to a temperature of 1200 ⁰ C and then from 1200 ° C with a temperature decrease of 10 ° C cooled h / h to ambient temperature slowly with a decrease.

The manganite crystals formed can then be separated manually from the other oxides, since the expansion coefficients are different and they easily peel off after deformation of the flat crucible. They are then freed from adhering oxides by washing with dilute hydrochloric or nitric acid. But you can also treat the contents or sediment of the crucible directly with dilute acid (HCl or HNO ₃ ) and get the manganite crystals formed by dissolving the other oxides.

Example 2

_{A homogeneous mixture of 30 mol Bi 2} O ₃ , 6 mol Mn ₂ O ₃ and 3MoI Y ₂ O ₃ (yttrium oxide) is melted in a platinum crucible at 1350 ° C. The solution is supersaturated by slow evaporation at 1410 ° C and until the complete evaporation of the Bi ₂ O ₃ - according to the second variant of the process - is kept at this temperature, ίο Finally, the temperature is slowly reduced by 10 ° C / h to ambient temperature cooled down.

Example 3

_{A homogeneous mixture of 20 mol Bi 2} O ₃ , 4 mol Mn ₂ O ₃ and 3 mol Er ₂ O ₃ (erbium oxide) is melted in a platinum crucible at 1350 ° C. The solution is slowly at a temperature of 1450 ° C until the complete disappearance of the

ao Bi ₂ O ₃ evaporated according to the second variant of the method according to the invention. Finally, the temperature is reduced by 10 ° C./h to ambient temperature.

Claims (5)

Patent claims: 1. A process for the production of single crystals of manganites rare earths by crystallization from a melt of the components in a solvent, characterized in that manganese oxide (Mn ₂ O ₃ ) and the oxide of a trivalent rare earth at a temperature above 1000 ° C in a - based on the manganese oxide - dissolves about five times the molar amount of bismuth oxide (Bi ₂ O ₃ ), which is known as a solvent for the single crystal growth of orthochromites of rare earths, the melt is initially oversaturated by slowly evaporating the bismuth oxide until the first manganite crystals appear in it, then in in a known manner with a cooling rate of about 4 to 10 ° C / h either cools immediately to ambient temperature or only after gradual complete evaporation of the bismuth oxide cools down and optionally separates the single crystals from the solidified melt in a known manner. 2. The method according to claim 1, characterized in that the dissolving of the oxides of the Manganese and the rare earth in the bismuth oxide at a temperature of about 1350 ° C he follows. 3. The method according to claim 1 or 2, characterized in that the solution of the Oxides by evaporation of bismuth oxide at a temperature of 1400 ° C or something over saturated. 4. The method according to any one of claims 1 to 3, characterized in that at the slow cooling of the solution, the temperature initially by about 4 ° C / h to one Value of about 1200 ° C lowers, from this temperature cools down by about 10 ° C / h. Considered publications:
»Chemie-Ing-Technik«, 28th year, 1956,
No. 5, p. 360, right column; J. Amer, Chem. Soc, 78: 4259 (1956). 509 597/340 6.65 © Bundesdruckerei Berlin