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 Rb_1−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.

Keywords:

• Relaxor
• RDP-ADP
• X-ray
• NMR-NQR
• Phase boundary

Acknowledgments

This work has been supported by Grand-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology in Japan. This work is also supported in part by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST).