PREPARATION, CRYSTAL-STRUCTURE AND SUPERCONDUCTIVITY OF (C,CU)(SR,CA)(2)(Y,CA,SR)CU2O7

A series of solid solutions containing CO3 groups, (CyCu1-y)(Sr1-z/2Caz/2)(2)(Y1-xCax-zSrz)Cu2Od with 0.30 less than or equal to x less than or equal to 0.70, was prepared at 1313 K under the atmosphere of O-2-CO2. The values of y(0.48-0.67) and z(0.14-0.28) depend on the total Ca content, x. The length of the a-axis increases from 3.8374(2) Angstrom (x=0.30) to 3.8591(9) Angstrom (x=0.70) with increasing x, while the c-axis length decreases from 11.1198(6) to 10.9410(6) Angstrom. All the as-prepared solid solutions are semiconductors, but superconducting transition is observed for the samples with 0.35 less than or equal to x less than or equal to 0.70 after annealing at 1273 K and an oxygen pressure of 4.0 MPa. The transition temperature (T-c) increases with increasing x, and the highest T-c=60 K is measured for the samples with 0.60 less than or equal to x less than or equal to 0.70. All the annealed samples show appreciable expansion of the c-axis length and contraction of the a-axis length relative to the as-prepared ones. The crystal structures of the as-prepared and annealed samples with x = 0.50 were refined from neutron powder diffraction data by means of the Rietveld method. On the basis of the refined crystal structure data and wet chemical analysis, it is suggested that hole doping is caused, during high-pressure oxygen annealing, by the release of some CO3 groups and by the introduction of oxygen atoms into the vacant ''apical'' oxygen site, where the CO3 group is not present.