Random fields influence on dynamic properties of disordered ferroelectrics

Abstract

The theoretical model for description of dielectric relaxation in highly polarizable dielectrics with dipole impurities like K_1–x Li _x TaO₃ (KTL) is suggested. In this model the distribution function of relaxation times is expressed through distribution function of random electric fields of impurity dipoles, which in turn calculated on the base of dipole-dipole interaction law only. The derived expressions for dynamic dielectric permittivity together with equation for long-range order parameter determine completely the relaxational properties of the system under consideration in whole range of temperatures and impurity content: in dipole glass state, in mixed ferroglass phase, in paraelectric and ferroelectric phases. It is shown that in dipole glass state relaxation has long-time logarithmic character, while in mixed phase the crossover between glassy and purely Debye dynamics, peculiar to paraelectric and ferroelectric phases occurs. It is shown also that Cole-Cole diagram in mixed phase has two archs and asymmetric shape in dipole glass state due to logarithmic relaxation. Latter features can be used in experiments for mentioned phases identification. Developed general formalism was used for quantitative description of experiments on frequency and temperature dependences of dielectric permittivity in KTL and further experiments were suggested.

Key Words:

• Ferroelectric
• electric dipole
• dielectric permittivity
• distribution function
• dipole glass