Cover illustration: Two-dimensional isostructurality among polymorphs of benzene-1,2-diamine, showing (top) the crystal packing in one polymorph, viewed down the a axis, and (bottom) the crystal packing in a second polymorph, viewed down the b axis. Hydrogen bonds are shown as dotted lines. See Czapik & Gdaniec [Acta Cryst. (2010), C66, o198-o201].
The title structure is a new modification of Tl2CrO4. It is closely related to the already known modification which belongs to the β-K2SO4 family, but it differs in the number of independent cations and anions: there are four independent Tl+ cations and two [CrO4]2− anions in the title structure, in contrast with the β-K2SO4 family where there are two independent cations and one anion.
The structure of the title compound, in the space group F23, is related to a cubic perovskite, with isolated RuO6 octahedra and distorted O polyhedra around Al. The La and Sr atoms share the same sites.
The monoclinic structure (C2/c) of the title compound is built up from layers of PO4 and ZnO4 vertex-sharing tetrahedra parallel to the bc plane with interlayer ZnO2(N/O)2 tetrahedra. Neighbouring layers are linked via Zn—O—P bridges. Hydrogen bonding and weaker electrostatic contacts are present between adjacent ZnO2(N/O)2 tetrahedra and between these tetrahedra and the layers parallel to the bc plane.
The Cu2Se3Sn structure investigated in this paper is a superstructure of the cubic and monoclinic modifications reported earlier. The eight independent Cu atoms and four independent Sn atoms occupy, respectively, one-third and one-sixth of the tetrahedral holes in a pseudo-close-packed Se lattice. The cation positions are fully ordered.