NEW 1212-TYPE SUPERCONDUCTORS WITH A T(C) UP TO 85-K IN THE SYSTEM HG-PR-SR-CA-CU-O

A new family of 1212-type superconductors, Hg0.4Pr0.6Sr2(Sr1-xPrx)Cu2O 6+delta (0.2 less-than-or-equal-to x 0.7) and Hg0.4Pr0.6Sr2(Ca1-xPrx)C u2O6+delta (0.2 less-than-or-equal-to x less-than-or-equal-to 0.7) has been synthesized. Electron-microscopy studies show that all these oxi des belong to the ''1212'' family; nevertheless, they differ from the classical ''1212'' mercury-substituted cuprates because of the orderin g between Pr and Hg in the [Hg1-xPrxOdelta]infinity layers. Two kinds of superstructures called O(I) (a(I) almost-equal-to 2a(p). b(I) almos t-equal-to a(p), c(I) almost-equal-to c1212 and O(II) (a(II) almost-eq ual-to 2a(p), b(II) almost-equal-to a(p) c(II) almost-equal-to 2c1212) are indeed observed. In both types, the Pr and Hg rows alternate alon g the a-axis, within each [Hg1-xPrxOdelta]infinity layer; two successi ve layers are in phase in the O(I) structure, wham they are shifted ov er a(p) in the O(II) structure, leading to a doubling of the c1212 par ameter. Evidence is also found for more complex cationic distributions responsible for the presence of diffuse streaking along c. All these oxides are superconducting for x less-than-or-equal-to 0.50; a T(c) u p to 8 5 K, a sharp transition and a superconducting volume fraction o f 100% are observed. The influence of the oxygen pressure during synth esis upon the superconducting properties of these materials is also st udied.