The Barium Oxotellurate(IV) Bromides Ba6Te10O25Br2and Ba3Te3O8Br2with Channel Structures

The calcium oxidotellurates Ca2(TeIVTeVIO7), Ca2(TeIVO3)Cl2 and Ca5(TeIVO3)4Cl2 obtained from salt melts

The mixed-valent calcium oxidotellurate(IV,VI) Ca2(Te2O7) (= Ca2(TeIVTeVIO7)) and the two calcium oxidotellurate(IV) dichlorides Ca2(TeIVO3)Cl2 and Ca5(TeIVO3)4Cl2 were obtained from metal chloride salt melts. Structure determination from single crystal X-ray data revealed a unique crystal structure in each case. A common motif in the three crystal structures is the formation of a framework perforated by channels into which the stereochemically active 5s 2 electron lone pairs E of the TeIV atoms are directed. Ca2(Te2O7) belongs to category I of the Robin-Day classification of mixed-valent compounds. Its crystal structure features a double chain of condensed trigonal-pyramidal [TeIVO3] and octahedral [TeVIO6] units embedded within a framework of [CaO x ] polyhedra (x = 7–8). The Cl atoms in the crystal structures of Ca2(TeIVO3)Cl2 and Ca5(TeIVO3)4Cl2 act as spacers between isolated trigonal-pyramidal [TeO3] units and exhibit weak interactions with the 5s 2 electron lone pairs E, with Cl⋯E distances <3.6 Å.

Mixed-valent 1:1 oxidotellurates(IV/VI) of Na, K and Rb: superstructure and three-dimensional disorder

Single crystals of three mixed-valent oxidotellurates(IV/VI) with general formula M 2TeIVTeVIO6 (H2O) x (M = Na, K, Rb) were grown under hydrothermal conditions at 210 °C from M 2CO3, TeO2 and Te(OH)6 in a 4:1:1 molar ratio. Na2Te2O6·11/8H2O (where TeIVTeVI is abbreviated as Te2) crystallizes as a monoclinic eightfold superstructure with occupational modulation of the water molecules. The structure is built of diperiodic corrugated [Te2O6]2− networks interleaved by the Na atoms and H2O molecules. The hexagonal crystal structures of M 2Te2O4(OH)4 (M = K, Rb; x = 2) with P63/mmc symmetry are made up of [Te2O4(OH)4]2− rods extending in the [001] direction. The rods are connected by the M atoms. The [TeIVO2(OH)2] units are disordered about the threefold axis, resulting in pronounced three-dimensional diffuse scattering.

The allotwinning of KCa3Te5O12Cl3: an OD interpretation

The crystal structure of the oxotellurate(IV) chloride KCa3Te5O12Cl3 is built up of [KTe5O12]3− layers sandwiched by layers of Cl− and Ca2+ ions. The structure can be considered as a twelvefold superstructure with occupational modulation of the K/Te positions. The electron lone-pairs of the Te(IV) atoms are directed towards the free space of the chloride layers. KCa3Te5O12Cl3 can be described as a category III order-disorder (OD) structure in terms of polar layers of one kind. The crystals under investigation are built up of the two polytypes of a maximum degree of order (MDO) with isomorphic C2/c and I2/a symmetry, both of which appear in two orientations. Two data reduction approaches, using either a reciprocal superlattice or three independent domains, were performed and the resulting refinements are comparatively discussed.