Crystal structure of (Hg, Ce, Cu)(Sr, Pr)(2)(Pr, Sr)Cu2O6+delta and (Hg,rectangle)(Sr, Pr)(2)(Pr, Sr)(2)Cu,O8+delta phases

This paper reports on the refined compositions and crystal structures of tw o phases from the homologous series (Hg, M)(Sr, Pr)(2)(Pr, Sr)(k-1)CukO2k+2 +delta with k = 2 (Hg-1212) - (Hg0.44(2)Ce0.314+Cu0.25)x(Sr0.90(3)Pr0.10)(2 )(Pr0.52Sr0.48)Cu2.00O7.00 (a = 3.8634(1), c = 12.2030(8) Angstrom, space g roup P4/mmm) with k = 3 (Hg-1223) - (Hg(0.29(1))square (0.71))(Sr0.67(2)Pr0 .33)(2)(Pr0.61(2)Sr0.39)(2)Cu3.00O9.32(8) or (Hg(0.29(1))square (0.71))x(Sr 0.61(4)Pr0.39)(2)(Pr0.51(4)Sr0.49)(2)Cu3.00O9.00 (with a fixed content of s uperstoichiometric oxygen O(4) in the phase) (square is a vacancy; a = 3.92 94(9), b = 3.8567(6), c = 15.2763(44) Angstrom, space group Pmmm). The impl ausibly high content of oxygen in the Hg-1223 phase (refinement I) is attri buted either to O-O bond formation ol to the possible presence of a minor a mount of copper in a defect position of Hg; the inclusion of the latter in structure refinement leads to a better reproduction of the real structure b ut increases the R indices. The crystal-chemical analysis of the title phas es and a comparison with the available data for analogous phases indicates that the composition of the crystallographic positions is related to struct ural features, in particular, to the coordinates of (Hg, M) and superstoich iometric oxygen, whose content depends on the degree of substitution of str ontium ions by praseodymium ions. Reasons for the orthorhombic distortion o f the Hg-1223 phase are discussed. The absence of superconductivity is expl ained by the nonoptimal formal charge (FC) of copper, which depends on the oxygen content in the phase.