The alkali metal copper(II) oxidotellurates(IV) Li₂Cu₂Te₃O₉, Li₂Cu₃Te₄O₁₂, Rb₂Cu₃Te₆O₁₆ and Cs₂Cu₃Te₆O₁₆ – four new structure types

Abstract

Four new MI/CuII/TeIV/O phases were obtained under hydrothermal conditions or with water as a mineralizer, and their crystal structures determined by single-crystal X-ray diffraction. Li₂Cu₂Te₃O₉ comprises trigonal-pyramidal [TeO₃], distorted tetrahedral [LiO₄] and distorted square-pyramidal [CuO₅] units, forming a framework penetrated by channels into which the electron lone pairs of the TeIV atoms are directed. Li₂Cu₃Te₄O₁₂ contains isolated trigonal-pyramidal [TeO₃] units, tetrahedral [LiO₄] groups forming [Li₂O₆] dimers by sharing an edge, as well as square-planar [CuO₄] and square-pyramidal [CuO₅] units, forming (Formula presented.) [Cu₃O₁₀] chains oriented parallel to [100]. These units also form a framework structure with channels where the electron lone pairs reside. Rb₂Cu₃Te₆O₁₆ and Cs₂Cu₃Te₆O₁₆ have isopointal crystal structures containing isolated [TeO₃] groups and (Formula presented.) [Te₄O₁₀] chains oriented parallel to [010]. While in Rb₂Cu₃Te₆O₁₆ both Cu sites are 4-coordinate in form of nearly square-planar [CuO₄] units, in Cs₂Cu₃Te₆O₁₆ one Cu site has a fifth oxygen contact resulting in a distorted square-pyramidal [CuO5] polyhedron. Both crystal structures are made up from two alternating layer types stacked along [100]. The differences between Rb₂Cu₃Te₆O₁₆ and Cs₂Cu₃Te₆O₁₆ originate almost exclusively from the layer type consisting of the 5-coordinate Cu site in Cs₂Cu₃Te₆O₁₆, while the other layer type shows almost no difference. A quantitative comparison between the two crystal structures is given.