Synthesis and Structural Characterization of New Phases in the Cubic M3Te2O6X2 (M = Sr, Ba; X = Cl, Br) Structure Family

Four alkaline earth oxotellurate(IV) halides with common formula M3Te2O6X2 (M = Sr, Ba; X = Cl. Br) have been prepared as polycrystalline powders and/or in the form of single crystals. All compounds crystallize in the cubic space group Fd (3) over barm with cell parameters a = 15.9351(4) angstrom for Sr3Te2O6Cl2 (single-crystal X-ray data), 16.052(5) angstrom for Sr3Te2O6Br2 (powder X-ray data), 16.688(2) angstrom for Ba3Te2O6Cl2 (single-crystal X-ray data) and 16.8072(3) angstrom for Ba3Te2O6Br1.64Cl0.36 (single-crystal X-ray data). The results of the crystal structure analyses reveal a rigid (3)(infinity)[M3Te2O6](2+) framework which can be described as being composed of regular octahedra of two types of chemically non-bonded M-6 octahedra that are capped by trigonal pyramidal [TeO3] anions located above every second face of one of the M-6 octahedra. The halide X- anions are situated in the voids of the (3)(infinity)[M3Te2O6](2+) framework. Dependent on the nature of the halogen, the anions show various kinds of occupational disorder which eventually led to a revision of the previous structure model of Ba3Te2O6Cl2. A comparative discussion with other structures of general formula M(3)Ch(2)O(6)X(2) (M= divalent metal; Ch = Te, Se; X = Cl, Br) is presented.