Phase transitions in elpasolites (ordered perovskites)

Many compounds with general chemical formula A(2)BB'X-6 and with tolerance factor value t < 1 crystallize in the elpasolite (ordered perovskite) struc ture. Different preparative methods have been considered. For example, the Bridgman technique is the most suitable method of growing single crystals o f halogenated elpasolites with monoatomic cations. Group-theory and symmetr y analyses have allowed us to build a scheme of possible structural distort ions connected with octahedral rotations and displacements of ions. Experim ental observations have proven that ferroelastic phase transitions occurrin g most frequently in elpasolites are due to small octahedra tilts. Quantita tive analysis allows the possibility of describing these transitions in the framework of the thermodynamic Landau theory. Hydrostatic pressure and the substitution of atoms in different crystallographic sites allow to change significantly the succession and the temperature of phase transitions. The hypothesis of bond stresses is able to predict the possible occurrence of a phase transition for a particular elpasolite and/or the role of the octahe dra rotations in the transformation. The mechanism of phase transitions in elpasolites and related compounds has also been considered. One can assume that a displacive mechanism associated with a strong anharmonicity plays a large role in phase transitions occurring in compounds with monoatomic cati ons. On the other hand, for instance, in cryolites with ammonium cations, t he transitions originate from an order-disorder phenomenon. (C) 1998 Elsevi er Science S.A. All rights reserved.