Study of the anisotropy of the dielectric response of Na$_{1/2}$Bi$_{1/2}$TiO$_3$ relaxor ferroelectric

The dielectric response, conductivity, and domain structure of (Na$_{1/2}$Bi$_{1/2}$)TiO$_3$ single crystals are studied in the temperature range of $290$–$750$ K for the $[100]$, $[110]$, and $[111]$ crystallographic directions. It is shown that the region of optical isotropization is observed in polarized light in the temperature range of $570$–$620$ K. In this case, the birefringence ($\Delta n$) decreases and disappears (together with the image of the domain structure) for the $[100]$ directions. The region of optical isotropization in the $[111]$ directions is characterized by the disappearance of the image of the domain structure and by the existence of individual regions with partial quenching. The domain structure in the $[110]$ directions remains distinguished against the background of a significant decrease in $\Delta n$ in the indicated temperature range. The region of isotropization is also manifested in the temperature dependence of the imaginary part of the dielectric response and is determined by the isotropic character of the conductivity in the range of $570$–$620$ K. The bulk conductivity has a thermally activated character with activation energies $E_a=50$–$60$ meV at $T<$ K and $E_a=700$–$900$ meV for $T>620$ K. The low-frequency dispersion of the dielectric response is determined by the Maxwell–Wagner mechanism and is due to an increase in the ionic conductivity at temperatures above $620$ K. The anisotropy of the susceptibility holds in the entire studied ranges of frequencies ($25$ Hz–$1$ MHz) and temperatures.