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

Abstract

Crystals of (Pb_1–xBa_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 ε_max and dielectric loss tangent tan δ_max 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, ε_max 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 U_a1 ≈ 0.6 eV and U_a2 ≈ 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.