Phase Formation Studies of Lead(II) Copper(II) Oxotellurates: The Crystal Structures of Dimorphic PbCuTeO5, PbCuTe2O6, and [Pb2Cu2(Te4O11)](NO3)2

Two modifications of the oxotellurate(VI) PbCuTeO₅ were isolated as single crystals from product mixtures obtained from solid state reactions, whereas single crystals of the oxotellurates(IV) PbCuTe₂O₆ and [Pb₂Cu₂(Te₄O₁₁)](NO₃)₂ were grown under hydrothermal conditions. The crystal structures of all compounds comprise of characteristic coordination polyhedra, viz. nearly square [CuO₄] plaquettes for divalent copper, octahedral [TeO₆] units for hexavalent tellurium, trigonal‐pyramidal [TeO₃] and bisphenoidal [TeO₄] groups for tetravalent tellurium, and distorted [PbO_x] polyhedra for divalent lead. PbCuTeO₅ is dimorphic and crystallizes in a monoclinic and a triclinic modification, related by a translationengleiche group‐subgroup relation of index 2. PbCuTe₂O₆ represents the ideal composition of the rare mineral choloalite. The characteristic feature of the crystal structure of [Pb₂Cu₂(Te₄O₁₁)](NO₃)₂ is its layered set‐up, comprised of cationic [Pb₂Cu₂(Te₄O₁₁)]²⁺ ribbons (width approximately 6.7 Å) sandwiched between nitrate anions that are only weakly bound to the cationic layers.