Crystal Growth of a Pb_1−xSc_xF_2+x Superionic Conductor and Investigation of the Interrelation between Its Thermal and Electrical Conductivities

A single crystal of a Pb_1−xSc_xF_2+x heterovalent solid solution with x = 0.1 (nominal composition) has been grown by vertical directional solidification (Bridgman technique), its elemental and phase compositions and crystallographic parameters have been determined, and the interrelation between its thermal and electrical conductivities has been analyzed. The composition of the solid solution has been found to vary from x = 0.08 in the bottom (cone) of the crystal to x = 0.095 in its top. The Pb_1−xSc_xF_2+x crystal has been shown to have low thermal conductivity (k = 0.7 W/(m K) at 300 K), with “glass-like” behavior of thermal transport, atypical of the crystalline state, in combination with high fluoride ion electrical conductivity (σ_dc = 0.012 S/m at 293 K) and relatively low activation enthalpy for electrical conduction (ΔH_σ = 0.378 ± 0.005 eV). The observed behavior of the thermal and electrical conductivities of the Pb_1−xSc_xF_2+x solid solution is due to structural disorder in the fluorine sublattice—which persists at room temperature—as a result of heterovalent substitutions of Sc³⁺ for Pb²⁺ cations. The thermal and electrical conductivities of single crystals of Pb_1−xSc_xF_2+x and Pb_1−xCd_xF₂ two-component solid solutions (CaF₂ structure) are compared to those of the β-PbF₂ (CaF₂ structure) and ScF₃ (ReO₃ structure) single-component fluorides.