Phase transitions and precursor phenomena in doped quantum paraelectrics

Abstract

Impurity-induced phase transitions in quantum paraelectrics like SrTiO₃:Ca²⁺ (SCT) are treated within the framework of a heterogeneous transverse Ising model. It is capable of describing both off-and on-center impurity systems in a realistic way. Owing to the local meanfield approach, however, the calculation fails to show the highly non-classic critical behavior observed in experiments. In addition, all fluctuation phenomena above the critical temperature are completely averaged out. In this precursor range fluctuating polar nanodomains are encountered, which give rise e.g. to relaxor-type polydispersive response, quasi first-order Raman scattering, bilinear coupling between transverse acoustic and polar soft-modes, and hyper-Rayleigh scattering which scales as ω _TOL ⁻⁴ . A novel SHG peak observed in SCT (x = 0.014) at T ≈ 40 K is attributed to hyper-Raman scattering at the difference mode of accidentally degenerate E_u and E_g soft-modes.

Key Words:

• Quantum paraelectrics
• ferroelectrics
• relaxors
• impurity induced phase transitions
• dielectric susceptibility
• Brillouin scattering
• SHG
• polar nanodomains