CRYSTAL-STRUCTURE OF THE SUPERCONDUCTOR CA2SR2CU3GAO9 SYNTHESIZED AT HIGH-PRESSURE

Ca2Sr2Cu3GaO9 with a T-c of 73 K was prepared at 1250 degrees C and 6 GPa. Its neutron powder diffraction data were analyzed on the basis of Ima2, Imcm and P2(1)ab space groups. Imcm gave the best approximation to the actual superstructure with a doubled b dimension. Ca2Sr2Cu3GaO 9 belongs to a homologous series of oxides (R(1-x)Ca(x))(n-1)Sr2CunGaO 2n+3 (R: rare-earth metals). In its structure, GaO4 tetrahedra form (G aO33-)(x) chains running parallel with the c-axis. Sr2GaO3 blocks comp osed of the infinite chains and Sr2+ ions alternate with (Ca)(2)(CuO2) (3) sheets along the a-axis. Two types of the infinite chains with dif ferent orientations of GaO4 tetrahedra alternate along the b-axis, whi ch is the reason for the 2b superstructure. The Cu(1)O-5 pyramid tilts owing to the tetrahedral coordination of Ga. The intermediate Cu(2)O- 2 sheet is nearly flat with each Cu(2) atom having four equivalent Cu- O bonds. Sr2+ and Ca2+ ions are completely ordered on two different si tes. Full occupation of Ca2+ ions at an eight-coordinated site, which is obtained only at high pressure, permits adequate hole doping of the CuO2 sheets, where the formal oxidation state of Cu is +2.33.