PHASE-TRANSITIONS IN THE COMPOUNDS ASBOSIO4 (A=NA, K) AND ASBOGEO4 (A=K, RB) .1. CRYSTAL-STRUCTURES OF THE PARAELECTRIC PHASES

Crystal structures of the KTiOPO4-like phases ASbOSiO4 (A = Na, K) and ASbOGeO4 (A = K, Rb), above the ferroelectric phase transition temper ature Tc, have been determined (space group Pnan). The atoms of the [S bOSiO4]- or [SbOGeO4]- framework show only small displacements upon Pn a2(1) to Pnan symmetry change at the transition. For the alkali ions A +, the displacements are much more pronounced, and in all the paraelec tric phases A+ becomes distributed over two equivalent sites in the ca vities defined by the framework. A precise description is given of the cavities in the title phases at T = 20-degrees-C and T > Tc, and in K TiOPO4 at T = 20-degrees-C, as well as of the A positions within these cavities. The phases transitions are displacive for the framework, an d of the order-disorder type for A, thus allowing the role of the alka li ions in the lowering of symmetry; leading to the ferroelectric phas es, to be underlined.