An aspect on relaxor behaviours and phase boundaries in ferroelectric mixed crystals

Abstract

In order to search for Pb-free relaxor substances and to define relaxor behaviour, a theoretical aspect is presented from not only microscopic but also phenomenological viewpoint. In order–disorder type mixed crystals, the microscopic approach is attempted under conditions similar to those for relaxor perovskite-type oxides. Basing on the theory for the precursor to glass state, two kinds of technique of X-ray diffuse scattering (static properties) and quadrupole perturbed nuclear magnetic resonance (NMR-NQR) (dynamic properties) are discussed in ferroelectric (FE) -- antiferroelectric (AFE) mixed crystals of Rb1−x(NH4)xH2PO4. After investigating both boundary regions between FE and glass (G) phases, and between AFE and G phases, relaxor-like behaviours as a large dispersion in dielectric constants are found in the AFE-G phase boundary region where the gradual growth of wave number to make the energy minimum and a discontinuous broad NMR-NQR spectrum appear in quite different forms from those of the FE-G phase boundary region. On the other hand, in perovskite-oxides, the conditions for relaxor appearance are discussed within the phenomenological theory by comparing with the microscopic analysis in hydrogen-bonded systems.