Dielectric properties of crystals of (Pb1–x Ba x )5Ge3O11 solid solutions

Abstract

Crystals of (Pb1–x Ba x )5Ge3O11 (0 ≤ x < 0.12) ferroelectric solid solutions have been grown via slow melt cooling and characterized by X-ray diffraction and dielectric measurements. The results demonstrate that the partial Ba substitution for Pb atoms in the crystals leads to a monotonic increase in their hexagonal cell parameters and a rather drastic degradation of their ferroelectric properties: a decrease in their Curie temperature T C from 450 to 120 K and a decrease and broadening of the peaks in their dielectric permittivity emax and dielectric loss tangent tan δmax at T C. In addition to the dielectric anomalies near T C, the dielectric properties of the crystals demonstrate features of relaxation character in the ranges 220–260 and 20–170 K. Starting in the composition range x = 0.06–0.08, emax increases with Ba content, instead of decreasing. Moreover, in this composition range the dielectric properties of the crystals depend markedly on their thermal history. The observed anomalies in the temperature and composition dependences of the dielectric properties of the solid solutions have been interpreted in terms of the dynamics of the thermal localization of charge carriers at defect levels with ionization energies Ua1 ≈ 0.6 eV and Ua2 ≈ 0.2 eV, as well as at shallower levels, in the temperature range 240–280 K and below 160 K, respectively, and the shift of T C to these temperature ranges with increasing Ba content.