Crystal Growth of a Pb1−xScxF2+x Superionic Conductor and Investigation of the Interrelation between Its Thermal and Electrical Conductivities

A single crystal of a Pb1−xScxF2+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 Pb1−xScxF2+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 (σdc = 0.012 S/m at 293 K) and relatively low activation enthalpy for electrical conduction (ΔHσ = 0.378 ± 0.005 eV). The observed behavior of the thermal and electrical conductivities of the Pb1−xScxF2+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 Pb1−xScxF2+x and Pb1−xCdxF2 two-component solid solutions (CaF2 structure) are compared to those of the β-PbF2 (CaF2 structure) and ScF3 (ReO3 structure) single-component fluorides.