Spontaneous unipolarity and anomalies of the dielectric and piezoelectric properties and electrical conductivity of initially heavily doped polydomain LiNbO$_3$ : Zn crystals

A significant spontaneous unipolarity in initially polydomain LiNbO$_3$ : Zn crystals doped in the range of “threshold” impurity concentrations ($\sim$ 4.5–4.7 mol%) has been revealed from high-temperature measurements and after high-temperature annealing under short circuit conditions. The effect of strong increase in the spontaneous unipolarity is accompanied by a pronounced low-frequency dielectric dispersion and abrupt anomalies in the temperature dependences of the electrical conductivity and dielectric permittivity. The piezoelectric coefficient $d_{333}$ increases to nearly maximum values available in the reference literature for nominally pure single-domain LiNbO$_3$ crystals.