IONIC CONDUCTIVITY AND THERMAL STABILITY OF BiF₃ CRYSTALS

Abstract

The temperature dependence of the ionic conductivity σdc(T) and thermal stability of BiF3 crystals with the structure of orthorhombic β-YF3 (sp. gr. Pnma, a = 6.5620(1) Å, b = 7.0144(1) Å, c = 4.8410(1) Å, V/Z = 55.71 Å3), grown from melt by the vertical directional crystallization technique have been investigated. The electrical characteristics of BiF3 are obtained from impedance measurements in the temperature range of 360−540 K. The σdc value at T = 500 K and the ion transport activation enthalpy ΔHa are found to be 2.5 × 10−5 S/cm and 0.48 ± 0.05 eV, respectively. The ΔHa value for the crystal studied is smaller by a factor of 1.4 in comparison with the isostructural rare-earth (Tb, Ho, Er, Y) trifluorides, which is due to the high electronic polarizability and large ionic radius of Bi3+ cations. It was found that BiF3 crystals are thermally stable at temperatures up to 550−600 K; at higher temperatures degradation was observed due to the sublimation and pyrohydrolysis of this material. The formation of oxofluoride phases is responsible for the detected conductivity jump in the dependence σdc(T) at T ∼ 600 K.