Polymorphism of CaTeO3 and solid solutions Ca(x)Sr(1-x)TeO3.

Single crystals and microcrystalline samples of the calcium tellurate(IV) phases alpha-, beta-, beta'- and gamma-CaTeO3 as well as of two solid solutions Ca(x)Sr(1-x)TeO3 (x = 0.55 and 0.77) have been synthesized and characterized by X-ray diffraction and thermal analysis. A comparative description of the structures and the relations between the polymorphs is given. The main building units of the hitherto unknown structures are isolated [Te(IV)O3]2- units and [(Ca,Sr)O(x)] (x = 6-8) polyhedra. All structures exhibit channels in which the Te(IV) electron lone pairs protrude. The low-temperature phase alpha-CaTeO3 is stable up to 1168 K. It exhibits nearly cylindrical channels (diameter approximately 4 A) and differs structurally from the other phases, whereas the metastable high-temperature phases are closely related to each other. They feature oval channels (shortest and longest diameter approximately 2 and 8 A). Gamma-CaTeO3 can be described as an order-disorder (OD) structure of two non-polar layers with layer groups p12(1)/m1 and p12(1)1. The gamma-CaTeO3 crystal under investigation consists of two polytypes with a maximum degree of order. The two phases Ca(x)Sr(1-x)TeO3 (x = 0.55 and 0.77) are isostructural to the MDO1 polytype of gamma-CaTeO3. Beta-CaTeO3 shows a distinct reversible phase transition at approximately 293 K. The low-temperature modification beta'-CaTeO3 as well as its high-temperature modification beta-CaTeO3 can be considered as threefold superstructures along [100] based on the MDO1 polytype of gamma-CaTeO3.