Perovskites

Perovskites allow for a huge amount of compositional and structural variation, giving rise to a multitude of interesting microwave dielectric properties. Simple perovskites like LaAlO3, NdAlO3, LaGaO3, and CaTiO3 possess high Q-factors, but most of them are far from being thermostable. The term "perovskite" is used to describe an arrangement of cations and an anion, with the generic stoichiometry ABX3, that is, isomorphous with CaTiO3. The anion octahedra are corner-shared, which is a key feature of all perovskites. The concept of the perovskite tolerance factor (t) was first suggested by Goldschmidt to indicate the stability of various perovskite structures for a given set of ions. By assuming that the cation and anion sublattices are closely packed, the tolerance factor can be viewed as a measure of distortion of oxygen octahedral. Colla et al. and Reaney et al. have shown that temperature stability in Ba- and Sr-based complex perovskites is fundamentally related to the onset and degree of octahedral tilting.