Abstract
Ca 2Sr 2Cu 3GaO 9 with a T c of 73 K was prepared at 1250°C and 6 GPa. Its neutron powder diffraction data were analyzed on the basis of Ima2, Imcm and P2 1ab space groups. Imcm gave the best approximation to the actual superstructure with a doubled b dimension. Ca 2Sr 2Cu 3GaO 9 belongs to a homologous series of oxides (R 1− x Ca x ) n−1 Sr 2Cu n GaO 2 n+3 (R: rare-earth metals). In its structure, GaO 4 tetrahedra form (GaO 3 3−) ∞ chains running parallel with the c-axis. Sr 2GaO 3 blocks composed of the infinite chains and Sr 2+ ions alternate with (Ca) 2(CuO 2) 3 sheets along the a-axis. Two types of the infinite chains with different orientations of GaO 4 tetrahedra alternate along the b-axis, which is the reason for the 2 b 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 CuO bonds. Sr 2+ and Ca 2+ ions are completely ordered on two different sites. Full occupation of Ca 2+ ions at an eight-coordinated site, which is obtained only at high pressure, permits adequate hole doping of the CuO 2 sheets, where the formal oxidation state of Cu is +2.33.
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