WO1990008106A1 - Superconducting lanthanide composition having a valence iv - Google Patents

Abstract

The invention relates to a family of copper oxydes with mixed valence which are derived from the perowskite structure. It has the following formula: Tlx?Try?Altz?CuO5-$g(d)? wherein Tr is a rare earth susceptible of having a valence IV; wherein Alt is an alkaline earth component selected in the group comprised of strontium and baryum; wherein x is higher than or equal to 1/3 and lower than or equal to 7/4, preferably higher than or equal to 0.5 and lower than or equal to 5/3; wherein y est lower than or equal to 1 and higher than or equal to 0; wherein z is lower than or equal to 2.5 and higher than or equal to 1/3; with x + y + z equal to about 3; wherein $g(d) is comprised between 0 and 1/3, preferably between 0 and 1/5. Application to the electrotechnical and electronic industries.

Description

 LANTHANIDE-BASED SUPERCONDUCTIVE COMPOSITION HAVING VALENCE IV.

The present invention relates to new copper oxides with mixed valence of structure derived from perowskite containing a lanthanide having a valence IV.

 It relates more particularly to superconductive compositions having at least partially a structure derived from perowskite.

 During the last luster, these oxides, which result from an intergrowth of a structure of the perowskite type and of a structure of the sodium chloride type, we have been the subject of very numerous studies, and all the more so their electrical and more particularly superconductive properties have been demonstrated at temperatures above 60 /

70 K.

 These copper oxides have structures generally described as oxygen deficient, which means that there is not enough oxygen to fill all the available sites of the perowskite. In fact, they have superstoichiometric zones of oxygen compared to divalent copper. This overstoichiometry is undoubtedly linked to their characteristic of being superconductive at relatively high temperatures.

 The structures of these copper oxide compounds are described in numerous publications. Among these, we should mention:

A) Article by B. RAVEAU "Many oxide structures can be constructed from the basic type ReO ₃ " - Proc. Indian Natn.

 Sci Acad., 52, A, n ° 1, 1986, pages 67-101;

 B) Article by L. ER-RAKHO, C. MICHEL, J. PROVOST and B.

RAVEAU "Perowskite series lacking oxygen containing CuII, CuIII" - Journal of Solid State Chemistry 37, 151-156 (1981);

C) Article by C. MICHEL and B. RAVEAU "Oxidation of oxygen in mixed valence copper oxides related to perowskites" - Revue de Chimie Minérale, t 29, 1984, page 407; D) Article by C. MICHEL, F. DESLANDES, J. PROVOST, P. LEJAY, R. TOURNIER, M. HERVIEU, B. RAVEAU, CRAcadémie des Sciences, tome 304, II, nº 17, p.1059 (1987) ;

 E) Article by Ce MICHEL, F. DESLANDES, J. PROVOST, P. LEJAY, R. TOURNIER, M. HERVIEU and B. RAVEAU, report to the Académie des

Sciences, tome 304, II, n ° 19, p.1169 (1987);

 F) The summary article, which in addition to the structural properties describes electrical and magnetic properties and which is published in Research No. 195 in January 1988, vol. 19, pages 53-60, entitled "The discovery of high temperature superconductivity" by K. Alex

MULLER and J. Georg BEDNORZ.

 Furthermore, such structures and their industrial applications have been, for example, described in patent applications whose objects are inventions made at the CRISMAT Laboratory in Caen, attached to the C.N.R.S., in particular those under the numbers 87 03717 and 87 03975.

 Over the past year, numerous sub-families of mixed valence copper oxide with a structure derived from perowskite have been studied, in particular with a view to obtaining a range of superconductors with specific and better adapted characteristics. than others to certain applications.

 The number of elements used in these structures and the variability of proportions lead to an almost infinite number of combinations. Only a very small fraction of these compounds has interesting electrical properties.

More particularly, we have sought to obtain perowskites whose zero resistance point (Tc ^{R = 0} ) is greater than the boiling point of liquid nitrogen. Considerable progress has been made when mixed valence copper oxides containing thallium have been discovered, their Tc ^{R = 0} sometimes being greater than 100K.

Compounds of the thallium family have been much studied because of this property of containing compositions whose critical temperature at zero resistance is greater than 100K. There are two main types of families of mixed valence copper oxides comprising in their system of thallium that of general formula:

Tl ₂ Ba ₂ C _{a m-1} Cu _m O _{2m + 4}

 and that of general formula:

TlA ₂ Ca _{m- 1} Cu _m O _{2m + 3} (with A = Ba and / or Sr).

 However, the first member of this last family, that is to say the one where m = 1, could never be synthesized and this although it would have been likely that it was particularly interesting due to the fact that all of the copper would be in "Cu III" form. Despite all the tests up to the present invention, no structure of this type could be produced to the knowledge of the applicant.

 This is why one of the aims of the present invention is to provide a new family which is of the structure described above, this aim has been achieved by copper oxides with mixed valence of structure derived from perowskite, characterized by the fact that 'they present the following formula:

Tl _x Tr _y Alt _z Cu O _{5- δ}

where Tr is a rare earth capable of having an IV valence, advantageously cerium and praseodymium, preferably praseodymium;

where Alt is an alkaline earth chosen from the group formed by strontium and barium;

where x is greater than or equal to 1/3 and less than or equal to 7/4, preferably greater than or equal to 0.5 and less than or equal to 5/3;

where y is less than or equal to 1 and greater than or equal to 0;

where z is less than or equal to 2.5 and greater than or equal to 1/3;

with x + y + z equal to about 3;

where δ is between 0 and 1/3, preferably between 0 and 1/5.

 It should be noted that Alt can be strontium, barium or a mixture of these two bodies.

The structure of the compounds according to the present invention is indicated in FIG. 1. This figure shows that the ideal limit structure, that is to say without crystallographic defect, consists of:

. a plane (1) of formula CuO ₂ ;

 . a plan (2) of formula AltO;

 . a plan (3) of formula TIO;

 . a plan (4) of formula AltO;

. then by a plan (! ') of formula CuO ₂ .

 Oxygen is symbolized by empty circles, brass by small black dots, alkaline earth by large black dots and thallium by a circle surrounding a multiplied sign.

 In the structure according to the invention, if x is less than 1, the plane (3) is mainly occupied by thallium and a part of the rare earth completes the available sites of this plane. The planes (2) and (4) are then occupied by the alkaline earth and the remaining rare earth. If x is greater than or equal to 1, the plane (3) is completely occupied by the thallium and the excess of thallium with respect to 1, if it exists, is distributed over the planes (2) and (4) with l alkaline earth and rare earth.

It should be noted that this structure corresponds to an intergrowth of a single layer of perowskite [(Sr, Pr, TI). CuO _{3 -δ} ] ∞and a double layer of sodium chloride type [(TI, Pr, Sr) ₂ O ₂ ] ∞.

 It should be emphasized that until the present invention no superconductive composition derived from perowskite did not contain rare earths capable of possessing a degree of oxidation IV such as praseodymium. Specialists in the field thought and demonstrated that in most structures the rare earths capable of presenting the valence IV and in particular praseodymium were a source of loss of superconductivity. This is the reason why the present invention leads to particularly interesting fields of application for perowskites.

Another object of the present invention is to provide a process for the synthesis of copper oxides with mixed valence according to the invention, this aim is achieved by means of the process for the synthesis of copper oxides with mixed valence comprising the following steps: a) grinding and mixing of thallic oxide (TI ₂ O ₃ ), strontium peroxide, barium peroxide, copper oxide and rare earth oxide in the desired proportion;

 b) compression of the mixtures obtained in a) in the form of pellets,

c) subjecting these pellets to heating at a temperature between 700 and 900 ° C, preferably between 750 and 850 ° C, in a tube sealed under primary vacuum (for example 10 ^-2 - 10 ^-3 mm Hg, or approximately 1

Pa) between 6 and 12 hours.

 d) slow cooling to room temperature.

 By slow cooling is meant natural cooling of the furnace (of the order of 2 to 6 hours).

The grinding is carried out so as to obtain a usual particle size in the material, such as a particle size having a d _{80 of} less than 20 micrometers, preferably 10 micrometers, generally around 5 micrometers. It is possible to work with even smaller grinding meshes by carrying out wet grinding, which is relatively favorable to the final compound but which requires a drying operation before step b) or during step a). This is why dry grinding is preferred. The compression of step b) is carried out under usual pressure in the material, such as compressions greater than 0.5 tonnes / cm ² (50.10 Pa) generally between 0.1 and 1.10 Pa.

 It is also possible to carry out this synthesis at non-atmospheric pressures. In these cases, the conditions on the oxygen content remain identical, expressed as partial pressure of oxygen.

 In this process the oxides can be replaced by any body which by thermal decomposition or chemical reaction gives the desired oxide.

The following nonlimiting examples show the characteristics of the new superconductors according to the present invention. Examples 1 and 2 and 3

Different mixtures corresponding to the general formula were prepared from Tl ₂ O ₃ , BrO ₂ , CuO and La ₂ O ₃ so as to make the different copper oxides of Examples 1 and 2. These mixtures are put into the form of pellets by compression under a pressure of the order of 5 tonnes / cm ² (0.5.10 Pa). The pellets thus obtained are brought to between 750 and 850 ° C. in a quartz tube sealed under vacuum for approximately 6 hours, then are cooled to ambient temperature by natural cooling of the oven (5 to 6 hours).

 The pellets are ground to a particle size between about 10 and about 20 micrometers.

The samples used for physical characterizations are prepared in the form of bars of 12 × 2 × 0.5 mm ³ , the powders with a particle size of between 10 and 20 micrometers, are pressed at 4 tonnes / cm ² (0.4.10 ⁹ Pa) and sintered for 4 to 5 hours under the same atmosphere as that used for the synthesis, and at the same temperature.

Resistive transition

 The method used is that of the four points: 1 contact at each end of the bar, 2 contacts on one of the faces. The contacts are made either by silver lacquer or by diffusion of indium in the frit by means of ultrasound.

 A current generator (model 103A - Adret Electronique) sends direct current to the sample

(5.10 ^{- 3} mA≤l ≤ 109.99 mA). The voltage between the center points is measured using a Keithiey digital multimeter with a resolution of 1 μV.

Magnetic measurements

 We use a Foner type vibrating sample magnetometer (model 155 from EGG) equipped with a cryostat and allowing work up to 4.2K. The sample is a sintered bar a few millimeters in length. It is fixed to the end of a rigid rod whose other end is fixed to the vibrator.

 The displacement of the sample, placed in a uniform magnetic field, in front of the captive coils, gives rise to an induced voltage at the displacement frequency.

 It is thus possible to determine the magnetic moment of the sample, as a function of the uniform magnetic field applied and of the temperature.

 The superconductive volume is measured at 4.2K by comparing the diamagnetic susceptibility observed with that which should be obtained for a completely diamagnetic sample (niobium as reference). Example 1

It was thus possible to synthesize superconductive phases of formula Tl _0.8 Sr _{1. 6} Pr _{0, 6} CuO ₅ which are decomposed as follows: (Tl _0. 8Pr _0.2 ) (Sr _{1 .6} Pr _{0 .4} ) CuO _{5- δ} (0≤ δ≤ 0.2). This structure has diamagnetism below 40K and 2% diamagnetism at 4K. Example 2

The synthesis of compound Tl was performed _{1, 5} Sr _1.5 Pr _{0, 5 5-} CuO _δ wherein for the synthesis of Tc R = 0 is near 40K and has a diamagnetic volume of 10% to 4. Measurements resistance of the sample as a function of temperature for an intensity of 3mA are shown in Figure 2. Note the very high slope of the curve above the critical point.

Example 3

Was also carried out the synthesis of compound Tl ₁ Ba _0.9 Pr _{6 0 5 5} CuO _δ having a zero resistance in the vicinity of 40K (Figure 3) and 15% diamagnetism at 4K. A mixed valence copper oxide derived from the perowskite structure has so far been described in which the preferred rare earth is cerium or praseodymium. However, this rare earth is very difficult to access and the use of its IV valence can involve syntheses relatively difficult to conduct.

 Superconductive compositions are now proposed which use rare earths which are easier to access than praseodymium and whose valence IV only exists in the transient state.

 This object and others which will appear subsequently are achieved by means of a mixed valence copper oxide according to claim 1, characterized in that:

Tr is lanthanum,

y is less than or equal to 1.5 and greater than or equal to 0,

z is less than or equal to 2.5 and greater than or equal to 1/3,

with x + y + z is about 3,

 δ is between 0 and 1 preferably between 0 and 1/5,

Alt can also be strontium, barium or a mixture of these two bodies.

 It should be noted that thallium can present valence III and valence I simultaneously. The structure of the compounds according to the present invention is essentially that of FIG. 1.

 We will not depart from the invention by replacing the lanthanum with a lanthanide or a mixture of lanthanides.

 The following nonlimiting examples show the characteristics of the new lanthanum superconductors.

 Examples 4 and 5

Different mixtures corresponding to the general formula were prepared from Tl ₂ O ₃ , BaO ₂ CuO and La ₂ O ₃ so as to make the different copper oxides of Examples 4 and 5.

The preparation of the samples and the measurements are carried out as previously for Examples 1 to 3. Example 4

It was thus possible to synthesize superconductive phases of formula Tl _1.25 Ba La _0.75 CuO _{5- δ} which are broken down as follows:

(Tl) (Tl Ba _0.25 La _{0, 75) 5-CuO δ.} This phase has diamagnetism below 35 K and 7% diamagnetism at HK.

 Example 5

We synthesized the compound Tl Ba _1.4 La _0.6 CuO _5-δ for which Te is close to 50 K and has a diamagnetic volume of 12% at 4K.

Claims

 1. Copper oxide with mixed valence derived from the structure of the perowskite, characterized in that it has the following formula I Tl _x Tr _y AIt _z Cu O _{5- δ} where Tr is a rare earth, advantageously likely to have a valence IV, preferably cerium and praseodymium; where Alt is an alkaline earth chosen from the group formed by strontium and barium; where x is greater than or equal to 1/3 and less than or equal to 7/4, preferably greater than or equal to 0.5 and less than or equal to 5/3 5 where y is less than or equal to 1 and greater than or equal to 0: where z is less than or equal to 2.5 and greater than or equal to 1/3; with x + y + z equal to about 3; where 6 is between 0 and 1/3, preferably between 0 and 1/5.  2. Mixed valence copper oxide according to claim 1, characterized in that:  Tr is lanthanum, y is less than or equal to 1.5 and greater than or equal to 0, z is less than or equal to 1.5 and greater than or equal to 1/3, with x + y + z is about 3,  δ is between 0 and 1 preferably between 0 and 1/5.  3. Process for the synthesis of copper oxides with mixed valence according to one of claims I and 2, characterized in that it comprises the following steps: a) grinding and mixing of thallic oxide (TI ₂ O ₃ ), strontium peroxide, barium peroxyide, copper oxide and rare earth oxide in the desired proportion;  b) compression of the mixtures obtained in a) in the form of pellets,  c) subjecting these pellets to heating at a temperature between 700 and 900 ° C, preferably between 750 and 850 ° C, for 6 to 12 hours in a sealed tube containing a small proportion of oxygen, d) rapid cooling to ambient temperature.