Crystal Structure of (Hg, Ce, Cu)(Sr, Pr)2(Pr, Sr)Cu₂O_6+δ and (Hg, $\square$)(Sr, Pr)₂(Pr, Sr)₂Cu₃O_8+δ Phases

Abstract

This paper reports on the refined compositions and crystal structures of two phases from the homologous series (Hg, M)(Sr, Pr)₂(Pr, Sr)_k-1Cu_kO_2k+2+δ with k = 2 (Hg-1212) — (Hg_0.44(2)Ce ⁴⁺_0.31 Cu_0.25)(Sr_0.90(3)Pr_0.10)₂(Pr_0.52(3)Sr_0.48)Cu_2.00O_7.00 (a = 3.8634(1), c = 12.2030(8) Å, space group P4/mmm) with k = 3 (Hg-1223) — (Hg_0.29(1) $\square _{0.71}$)(Sr_0.67(2)Pr_0.33)₂(Pr_0.61(2)Sr_0.39)₂Cu_3.00O_9.32(8) or (Hg_0.29(1) $\square _{0.71}$)(Sr_0.61(4)Pr_0.39)₂(Pr_0.51(4)Sr_0.49)₂Cu_3.00O_9.00 (with a fixed content of superstoichiometric oxygen O(4) in the phase) ($\square$ is a vacancy; a = 3.8294(9), b = 3.8567(6), c = 15.2763(44) Å, space group Pmmm). The implausibly high content of oxygen in the Hg-1223 phase (refinement I) is attributed either to O–O bond formation or to the possible presence of a minor amount 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 but increases the R indices. The crystal-chemical analysis of the title phases and a comparison with the available data for analogous phases indicates that the composition of the crystallographic positions is related to structural features, in particular, to the coordinates of (Hg, M) and superstoichiometric oxygen, whose content depends on the degree of substitution of strontium ions by praseodymium ions. Reasons for the orthorhombic distortion of the Hg-1223 phase are discussed. The absence of superconductivity is explained by the nonoptimal formal charge (FC) of copper, which depends on the oxygen content in the phase.