Structure and crystal chemistry of the high-T_c superconductor YBa₂Cu₃O_7−x

Abstract

Materials within the phase diagram Y₂O₃–BaO–CuO have recently generated great interest because of the observation of superconductivity with a critical temperature (T_c ) of ≳90 K^1,2; the current consensus is that YBa₂Cu₃O_7−x (x ≈ 0.2) is the superconducting phase (see, for example ref. 3). Hazen et al.⁴ concluded, from a single-crystal X-ray diffraction experiment, that its structure is based on the corner-linked octahedral perovskite structure: the tetragonal unit cell (space group P&4bar;m2) arises from a tripling of the c-axis resulting from the ordering of yttrium and barium cations and associated oxygen defects. Here we present high-resolution neutron powder diffraction data for this phase with x = 0.15(7). Our results agree with ref. 4 with respect to cation positions, but the location of one set of oxygen defects is substantially different and necessitates the lowering of lattice symmetry from tetragonal to orthorhombic (space group Pmmm), in agreement with separate findings^5–8. In contrast to the 35-K superconductor, La_1.85Ba_0.15CuO₄, the corner-linked CuO₄ planar groups are connected not only as sheets in the a–b plane but also as chains parallel to the ft-axis. The average copper valence is 2.23 (5). From bond valence arguments we infer that the Cu²⁺ and Cu³⁺ ions preferentially occupy square pyramidal and square planar sites respectively. Crystal chemical considerations suggest that these structural features may be involved in the superconducting mechanism.