Anhydrous 3,4,5-trihy-droxy-benzoic acid, C(7)H(6)O(5), is essentially planar, with its non-H atoms exhibiting mean and maximum deviations from coplanarity of 0.014 and 0.0377 (5) Å, respectively. The C-C-C-OH torsion angle about the bond linking the carboxyl group to the benzene ring is -0.33 (10)°. In the crystal, the -COOH groups form centrosymmetric hydrogen-bonded cyclic dimers [graph set R(2) (2)(8)] and the phenolic -OH groups participate in both intra- and inter-molecular hydrogen bonds, forming a three-dimensional network structure.

A twin-free single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ has been studied at room temperature and 115 K by x-ray-diffraction analysis. The crystal was grown from an Y-Ba-Cu-O melt in a gold crucible, leading to a 2% substitution of Au for Cu. Twins were removed from the as-grown crystal by a novel thermomechanical process, resulting in an orthorhombic (Pmmm), single-domain crystal. Oxygen positions and occupancies in the Cu-O basal plane have been refined, showing that the O(5) site is completely vacant, and the O(4) atoms are offset from the crystallographic mirror plane positions by 0.15 Å in a zig-zag fashion. Gold, which is a common impurity in crystals grown by the present technique, was found to occupy Cu(1) sites only. Weak superlattice reflections suggest a possible three-dimensional ordering of O and/or Au. To our knowledge, this paper reports the first high-resolution x-ray study of an intentionally detwinned, superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ single crystal.