Order-disorder (OD) structures of Rb₂Zn(TeO₃)(CO₃)·H₂O and Na₂Zn₂Te₄O₁₁

Abstract

Single crystals of the two alkali metal zinc oxidotellurates(IV), Rb₂Zn(TeO₃)(CO₃)·H₂O and Na₂Zn₂Te₄O₁₁, were obtained by reactions of mixtures of ZnO, TeO₂, Rb₂CO₃ (molar ratios 2:3:6) and ZnO, TeO₂, Na₂CO₃ (molar ratios 2:3:10), respectively, with small amounts of water as a mineralizer. Both compounds crystallize as order-disorder (OD) structures of layers and feature a high stacking fault probability. The crystal structure of Rb₂Zn(TeO₃)(CO₃)·H₂O is composed of layers extending parallel to (100). The structure is composed of two kinds of non-polar OD layers consisting of trigonal-pyramidal [TeO₃]2-, tetrahedral [ZnO₄]6-, Rb1+, and CO₃2-, H₂O, Rb2+, respectively. Different centrings of the layer groups lead to an ambiguity in the stacking arrangement. The crystal structure of Na₂Zn₂Te₄O₁₁ is built from layers extending parallel to (001). Trigonal-pyramidal [TeO₃]2- and bisphenoidal [TeO₄]4- polyhedra form [Te₄O₁₁]6- groups, which are connected by longer Te-O-contacts to form 1∞[Te₈O₂₂]12- double chains oriented along either [100] or [010]. These chains form non-polar layers, which appear alternatingly in two orientations related by a fourfold rotoinversion. The Zn2+ and Na+ cations are located at the layer interface. The stacking ambiguity is due to different lattices of adjacent layers.