Effects of pressure on the dielectric properties and phase transitions of the alkali metal tantalates and niobates

Abstract

The hydrostatic pressure dependences of the static dielectric constants and phase transitions of the mixed perovskite crystals KTa0.68Nb0.32O3 (KTN) and K0.5Na0.5NbO3 and of LiTaO3 and LiNbO3 were investigated. For the mixed perovskites the pressure responses are qualitatively similar to those observed earlier for pure perovskites, and they can be understood in terms of the soft mode concept of displacive ferroelectric transitions. These mixed crystals exhibit with decreasing temperature the same sequence of phase transitions as in pure KNbO3 and the isomorphous BaTiO3, namely cubic to tetragonal, to orthorhombic to rhombohedral. Their pressure-temperature phase diagrams show that the temperature ranges of stability of the tetragonal and orthorhombic phases decrease with increasing pressure and that these phases should vanish at sufficiently high pressure. The cubic paraelectric to tetragonal ferroelectric transitions in these crystals are first order but they tend towards second order with increasing pressure. The pressure responses of LiTaO3 and LiNbO3 are qualitatively different from those of the perovskites. The ferroelectric transition in these crystals is believed to have both displacive and order-disorder (of the Li ion) character, but the pressure results suggest that the order-disorder transition is the dominant feature.