Rare earth substituted thallium-based superconductors

Abstract: The present invention comprises novel superconducting materials exhibiting superconductivity at temperatures exceeding 90 K with chemical formula Tl a Pb b Ca c Sr d R e Cu f O g where R = Y or the lanthanide rare earth elements and where 0.3≦a,b≦0.7,0.05≦c≦1.1,2-c≦d≦1.95,0.05≦e≦1,1.9≦f≦2.1 and 6.5≦g≦7.5. These compounds, which are layered perovskite-like oxides, exhibit a high chemical stability, form readily into nearly single phase, do not require adjustment of oxygen stoichlometry after synthesis and compositions may be chosen allowing superconductivity at temperatures exceeding 100K. A preferred form of the superconducting materials is the range of compositions given by the chemical formula Tl 0.5+x Pb 0.5-x Ca 1-y Sr 2-z R y+z Cu₂O 7±δ where O≦x,8≦0.3, O≦y≦0.95, 0.05≦z≦1-y of which a preferred form is Tl 0.5+x Pb 0.5-x Ca 0.8+x+z Sr 2-z R 0.2-x Cu₂O 7±δ exemplified by Tl 0.5 Pb 0.5 CaSr 1.8 La₀₋₂Cu₂O 7±δ where O≦δ≦0.1.

Description: The present invention relates to rare earth substituted thallium-based superconductors. A variety of high-Tc superconducting oxides have been discovered having superconducting transition temperatures Tc>77K (see R J Cava, Science 247 (1990) 656). These offer the possibility of commercial applications because of the ease and cheapness of liquid nitrogen refrigeration compared to liquid helium refrigeration as used by older conventional superconductors. Potential applications include magnets, magnetic screening, wires and cables for electrical power transmission, transformers, switches, current limiters, generators, motors, squids, electronics applications, mechanical bearings, medical body scanners and many others. These superconducting oxides include: YBa 2 Cu 3 O 7-δ (1-2-3), Y 2 Ba 4 Cu 7 O 15-δ (2-4-7), YBa 2 Cu 4 O 8 (1-2-4), Bi 2 CaSr 2 Cu 2 O 8 , Bi 2 Ca 2 Sr 2 Cu 3 O 10 , Tℓ m CaSr 2 Cu 2 O 6+m and Tℓ m Ca 2 Sr 2 Cu 3 O 8+m where m=1 or 2, and Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 . Of these, all except 1-2-4 and Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 require oxygen loading or unloading following synthesis in order to optimise the superconductivity for maximum Tc. This is done by annealing at an appropriate temperature and oxygen partial pressure or by electrochemical insertion, for example. This annealing step is an added complication in the manufacturing process and for bulk, dense material the oxygen diffusion coefficient is sufficiently low that oxygen loading or unloading is prohibitively slow (see J L Tallon et al, Applied Physics Letters 52 (1988) 507). The loading or unloading of oxygen is also instrumental in causing microcracking and it would be preferable to remove this step altogether (see D R Clark et al, J. Amer. Ceramic Soc. 72 (1989) 1103). The said compounds which do not require oxygen loading, 1-2-4 and Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 , have transitions of 80K and 83K respectively, too low to be useful at liquid nitrogen temperature, 77K. All of these compounds which incorporate Ba tend to be chemically unstable. 2-4-7, 1-2-4 and Bi 2 Ca 2 Sr 2 Cu 3 O 10 all have slow synthesis kinetics and require several days to synthesize. Bi 2 Ca 2 Sr 2 Cu 3 O 10 , and the Tℓ compounds all are difficult to prepare as single phase material as shown by figure 1a in the Example 1. The one material which does not require adjustment of oxygen stoichiometry (see J L Tallon et al, Physica C 161 (1989) 523), is chemically stable, and is rapidly synthesized (in a period of about two hours) is Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 . This compound however is not so easily synthesized as a single phase and the Tc value is about 83K. Liu et al , Japanese Journal of Applied Physics, (1989) 28 : L1179 - L1181, disclose single phase Tl 0.5 (Ca 1-x M x )Sr 2 Cu 2 Oy having a bulk superconductivity of up to 101 K and a tetragonal crystal structure. The present invention provides novel rare earth substituted superconductors the chemical formula for which may be generally expressed as Tl 0.5 Pb 0.5 CaSr 2-z La z Cu 2 O 7 with 0.15 ≤ z ≤ 0.3. In the material Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 the Tc value is low because the mean valence of Cu is too high, ie the hole carrier concentration is too high. The superconducting cuprates appear to have a general phase diagram which shows Tc as a function of hole concentration rising from zero, passing through a maximum then falling back to zero. It was believed that by reducing the hole concentration Tc in this compound should rise to a maximum and the stability of the compound should increase as the mean Cu valence is reduced towards the value 2. It is known for the superconducting cuprates based on the perovskite structure that, in general, a rare earth element R, or Y, will substitute for Ca when Ca occupies a perovskite site with oxygen vacancies on the neighbouring sites in the a-b plane. Rare earth elements are also known to partially substitute for Sr and Ba in perovskite A-sites. In general Y and the rare earth elements of smaller atomic radius will substitute predominantly for Ca while the rare earth elements of larger atomic radius will substitute predominantly for Sr or Ba in a perovskite lattice. In the present invention, the hole concentration is reduced in Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 by substituting La 3+ in place of Sr 2+ giving the compounds Tℓ 0.5 Pb 0.5 CaSr 2-z Cu 2 O 7+δ and as a consequence Tc is raised to a maximum over 100K. At the same time the stability of the compound is evidently increased as the substituted compound is readily synthesized as a single phase. Preferred compounds have z = 0.2. The compounds of the invention exhibit high chemical stability in contrast to compounds containing Ba, form readily into a single phase in contrast to the material Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 for example, do not require adjustment of oxygen stoichiometry after synthesis, and compositions may be chosen allowing superconductivity at temperatures exceeding 100K. The novel compounds described herein have the same tetragonal layered perovskite-like structure of the parent compound Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 comprising in sequence: a Tℓ 0.5 Pb 0.5 O layer with Tℓ/Pb occupying square comer-shared sites and oxygen distributed about the face centre; a SrO layer with oxygen occupying the square comer-shared sites and Sr occupying the face-centre; a square planar CuO 2 layer with Cu occupying the comer-shared sites and oxygen the edge-shared sites; a Ca layer with Ca occupying the face centre and oxygen vacancies at the comer-shared sites; another CuO layer and finally another SrO layer to complete the cell. Materials of the invention and of comparative examples will be further described with reference to drawings and the examples which follow. In the drawings that are referred to in the examples: Figure 1 shows the x-ray diffraction patterns, with impurity peaks marked by dots, for (a) Tℓ 2 Ca 2 Sr 2 Cu 3 O 10 prepared from a sample of nominal compound TℓCa 3 SrCu 3 O Y ; (b) a sample of nominal compound Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 ; and (c) a sample of compound Tℓ 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 . This sample is very nearly single phase with only two small impurity peaks. Compounds (a) and (b) are for comparison purposes only. Figure 2 shows the d.c. resistivity and a.c. susceptibility for a pellet of Tℓ 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 as a function of temperature. Figure 3 shows T c (R=0) plotted against z for the compounds Tℓ 0.5 Pb 0.5 CaSr 2-z La z Cu 2 O 7 (the invention as claimed requires 0.15 ≤ z ≤ 0.3). Example 1 Tℓ 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 was prepared by first synthesizing CaSr 1.8 La 0.2 Cu 2 O 7 using stoichiometric proportions of Ca(NO 3 ) 2 , La 2 O 3 , and CuO which were mixed and milled then decomposed at 700°C in air. The result was ground, milled and die-pressed into pellets and reacted at 820°C for 16hrs in air. The resultant ceramic was ground, milled and mixed with stoichiometric proportions of Tℓ 2 O 3 and PbO, milled again and pressed as 13mm diameter pellets. A pellet was placed in a gold bag in an atmosphere of oxygen, sealed by crimping and reacted for two hours at 950°C in flowing oxygen. The gold bag was air-quenched upon removal and standard four terminal resistivity and a.c. susceptibility measurements were made followed by x-ray powder diffraction analysis. Figure 1c shows the sample to be very nearly single phase Tℓ 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 in contrast to the unsubstituted compound in which impurity phases are usually found. Figure 1b shows an x-ray diffraction pattern for Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 (not being an embodiment of the invention), which illustrates the typical presence of impurity reflections marked by dots. The pattern for Tℓ 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 may be fitted on a tetragonal cell with lattice parameters a=0.3810nm and c= 1.2106nm. The temperature dependence of the resistivity and a.c. susceptibility for this compound shown in Figure 2 reveal that zero resistance and diamagnetism set in at 102K nearly 30K higher than for the unsubstituted compound Tℓ 0.5 Pb 0.5 CaSr 2 Cu 2 O 7 (not being an embodiment of the invention). No effects on Tc were observed arising from subsequent annealing over a range of oxygen partial pressures of temperatures. Thus, contrary to most of the other compounds cited above, the oxygen stoichiometry is fixed and Tc is maximised solely by adjusting z, the amount of La substituted onto the Sr-site. Examples 2 to 11 The series of materials Tℓ 0.5 Pb 0.5 CaSr 2-z La z Cu 2 O 7 , for z=0.1, 0.15, 0.20, 0.25, 0.3, 0.4, 05, 0.6, 0.7, 0.8 respectively, were prepared in a similar manner to example 1, but beginning by synthesizing CaSr 2-z La z Cu 2 O 5 (materials having z=0.4 to 0.8 inclusive are for comparison purposes only). Pellets of each material were annealed in open gold boats for one hour at 700°C in flowing oxygen and cooled to 200°C over two hours in order to sharpen the transition to the superconducting state, as measured by a.c. susceptibility. The materials were each shown to be nearly single phase by x-ray powder diffraction, and the patterns were all fitted on a primitive tetragonal unit cell. Values of the superconducting transition temperatures and lattice parameters for all the examples are listed in Table 1. Superconducting transition temperatures, measured by the onset of diamagnetism, and tetragonal cell parameters for the series of materials Tℓ 0.5 Pb 0.5 CaSr 2-z La z Cu 2 O 7 Example z T c (K) a(nm) c(nm) 2 0.1 98.5 .3800 1.2095 3 0.15 106 .3805 1.2097 4 0.2 107 .3809 1.2095 5 0.25 106 .3810 1.2090 6 0.3 106 .3812 1.2082 7 0.4 94 .3815 1.2073 8 0.5 96 .3823 1.2060 9 0.6 83 .3826 1.2052 10 0.7 59 .3826 1.2020 11 0.8 33 .3832 1.1962 The foregoing describes the invention including preferred forms and examples thereof. The preparation of derivative materials for forms other than sintered ceramic form, ie thin films, thick films single crystals, filaments and powders other than those specifically exemplified will be within the scope of those skilled in the art in view of the foregoing whether by plasma, vapour, liquid, sol gel or solid state deposition and reaction, for example. The scope of the invention is defined in the following claims.

Claims: A material which exhibits high temperature superconductivity at above 100 K, having the formula Tl 0.5 Pb 0.5 CaSr 2-z La z Cu 2 O 7 , wherein 0.15 ≤ z ≤ 0.3. A material according to claim 1 having the formula Tl 0.5 Pb 0.5 CaSr 1.8 La 0.2 Cu 2 O 7 .

Inventors: PRESLAND MURRAY ROBERT, TALLON JEFFREY LEWIS, PRESLAND, MURRAY ROBERT, TALLON, JEFFREY LEWIS
Applicants: UNITED KINGDOM GOVERNMENT, HER MAJESTY THE QUEEN IN RIGHT OF NEW ZEALAND
Application year: 1991
Main classification: H01L39
IPC codes: C04B35, H01L39