PREPARATION AT MODERATE PRESSURES (P-LESS-THAN-OR-EQUAL-TO-20 KBAR) OF (SR,CA)CUO2 PHASES WITH INFINITE-LAYER STRUCTURE - SUPERCONDUCTIVITY AT 34 K IN (SR0.75CA0.25)(0.85)ND0.15CUO2

The composition versus pressure phase diagram of the system Sr1-xCaxCuO2 (0 less than or equal to x less than or equal to 0.85) has been studied by working at moderate pressures, up to 20 kbar, in a pyston-cylinder machine which enables one to prepare relatively large amounts of sample, of several hundreds of milligrams. Compounds with the ''infinite-layer'' (IL) structure can be prepared in this range of pressures with a maximum Sr/Ca ratio of 0.75/0.25. The crystal structure of these phases, refined from X-ray powder diffraction data by the Rietveld method, is consistent with the defect-perovskite tetragonal model established for the IL compounds. Due to the existing mismatch between CuO2 and (Sr, Ca) layers, the Cu-O bonds are under tensile stresses favoring the electron doping of these phases, which can thus be achieved under moderate pressures. At 20 kbar we prepared the IL compound (Sr0.75Ca0.25)(0.85)Nd0.15CuO2, for which the variation of unit-cell parameters shows that the CuO2 layers have been effectively electron doped. The as-prepared sample is superconducting at 34 K, showing a low Meissner volume of about 1%. The origin of such a small superconducting fraction is discussed in terms of the hypothetical presence of excess oxygen perturbing the perfection of the CuO2 layers.