Dipole defect decay and dielectric relaxation in Na0.5Bi0.5TiO3 single crystal
Electrical properties of heat treated in air and in vacuum Na0.5Bi0.5TiO3 single crystals were measured in AC field (f=1 kHz) in the range 300–800 K. Relaxation anomaly of permittivity ε(T) is observed near 700 K for the crystal heat treated in vacuum. It is argued that processing in vacuum generates the defect dipoles, which give rise to the dielectric anomaly. The character of the ε(T) experimental dependence is quite different from the predictions of Debye model. The non-Debye behavior of ε(T) is explained by the concentration decrease of dipole defects due to their thermal destruction at high temperatures. In addition, the ε(T) anomaly could be described more precisely by taking into consideration configurational and vibrational entropy of the subsystem of dipole defects. The observed dielectric relaxation is presumably attributed to reorientations of the dipole moments of Ti3+–VO centers.