Abstract
Pb2BTeO6 (B = Co, Mn, Cd, Ca) and Pb7.47Sr3.53Te4O23 were synthesized by solid state reactions, starting from TeO2, and precursors of lead(II) oxide and the respective metal oxides. Structure determinations from single crystal X-ray data at room temperature revealed that Pb2CoTeO6 and Pb2CoTeO6 adopt the B site ordered cubic double perovskite aristotype (Z = 4, Fm3¯m) whereas Pb2CdTeO6 and Pb2CaTeO6 crystallize as hettotypes thereof, with a reduction of the space group symmetry to C2/c and P21/c, respectively. Differential scanning calorimetry and temperature-dependent X-ray powder diffraction revealed reversible phase transitions around 270 °C for Pb2CdTeO6 and 225 °C for Pb2CaTeO6 to the cubic aristotype as the high-temperature phase. Pb7.47Sr3.53Te4O23 is a representative of a solid solution M11Te4O23 (M = Pb, Sr) with Sr2+ and Pb2+ cations occupationally disordered at the same sites in the cubic structure (Z = 8, Fm3¯m). Its crystal structure can be described as an eightfold superstructure with all axes doubled in relation to the fully ordered double perovskite structure, and with cation vacancies at one of the B sites, anion vacancies at one of the O sites, and an additional O site in the structure. The relationships between the aristotype structure and the different hettotypes (Pb2CdTeO6, Pb2CaTeO6 and M11Te4O23 (or M2.75TeO5.75 with Z = 32)) are concisely described with the aid of group-subgroup relations.
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