Mixed-valence Sm-doped LaF3 crystals as ion-electron conductors: Crystal growth and impedance characterization

The single crystals with the composition La_1−y(Sm³⁺_1−xSm²⁺_x)_yF_3−xy $L{a}_{1-y}{\left(S{m}_{1-x}^{3+}S{m}_x^{2+}\right)}_y{F}_{3-\mathrm{xy}}$(y = 0.04) were grown from melt by the vertical Bridgman technique. A part of the Sm³⁺ doping ions in LaF₃ matrix is reduced to the Sm²⁺ oxidation state due to interaction with carbon during the growth process. The crystals were studied by X-ray diffraction analysis, optical and impedance spectroscopy. The Sm-doped crystals LaF₃ are single-phase, retaining the tysonite-type structure (sp. gr. P-3c1$P\bar{3}c1$) and demonstrate a bipolar electrical conductivity mechanism. Both the ionic conductivity σ_i = 4.7 × 10⁻⁵ S/cm caused by heterovalent substitutions of La³⁺ for Sm²⁺ and the comparable electronic conductivity σ_e = 3 × 10⁻⁵ S/cm due to the variable oxidation states Sm²⁺/Sm³⁺ ions were detected for the grown crystals. The discovered mixed ionic-electronic conductivity of La_0.96Sm³⁺_0.004Sm²⁺_0.036F_2.964 crystals opens up a new direction for the practical application of the tysonite-type fluorides as a component of electrode materials for fluorine-ion current sources.