Electrical properties of bismuth phosphate and bismuth germanate single crystals

Electrical and dielectric properties, ionic transference numbers, thermal conductivity, and microhardness of Bi5.8PO11.2 (BPO) and Bi4Ge3O12 (BGO) single crystals are studied. Pr3+ and Er3+ are used as dopants in BGO. BPO crystals are mixed electron-ionic conductors (up to 500 °C, ti = 0.14 ± 0.01). The ionic conductivity, σ, and relative static permittivity, εs, are slightly anisotropic, their highest values, σ = 6 · exp (− 0.64 eV/kT) and εs = 50 ± 1, are found along the growth direction [1¯01]. BGO crystals have a low conductivity (1.5 10− 6 S/cm, at 600 °C) of a dominant ionic character (ti = 0.83 ± 0.02). Doping with rare earth ions decreases the conductivity significantly. The static relative permittivity of both “pure” and doped BGO is equal to 16.4 ± 0.1. The thermal conductivity and microhardness of BGO are equal to 4.5 ± 0.1 W/Km or 5.5 ± 0.1 kN/mm2, respectively. A slight impurity hardening takes place.