Crystal structure of the superconductor Ca2Sr2Cu3GaO9 synthesized at high pressureстатья
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Аннотация: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. Ca2Sr2Cu3GaO9 belongs to a homologous series of oxides (R(1-x)Ca(x))(n-1)Sr2CunGaO2n+3 (R: rare-earth metals). In its structure, GaO4 tetrahedra form (GaO33-)(x) chains running parallel with the c-axis. Sr2GaO3 blocks composed 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 different orientations of GaO4 tetrahedra alternate along the b-axis, which 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 sites. 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.