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

The single crystals with the composition La 1− y (Sm 3+ 1− x Sm 2+ x ) y F 3− xy ( y = 0.04) were grown from melt by the vertical Bridgman technique . A part of the Sm 3+ doping ions in LaF 3 matrix is reduced to the Sm 2+ 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 LaF 3 are single-phase, retaining the tysonite-type structure (sp. gr. P 3 ̄ c 1 ) and demonstrate a bipolar electrical conductivity mechanism. Both the ionic conductivity σ i = 4.7 × 10 −5 S/cm caused by heterovalent substitutions of La 3+ for Sm 2+ and the comparable electronic conductivity σ e = 3 × 10 −5 S/cm due to the variable oxidation states Sm 2+ /Sm 3+ ions were detected for the grown crystals. The discovered mixed ionic-electronic conductivity of La 0.96 Sm 3+ 0.004 Sm 2+ 0.036 F 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. • Sm-doped LaF 3 crystals P 3 ̄ c 1 grown from a melt by the Bridgman technique. • The crystals have been studied by X-ray diffraction, optical and impedance spectroscopy . • The crystals demonstrate a ionic-electronic conductivity. • The mixed-valence Sm-doped fluorides can be a component of electrode materials for fluorine-ion current sources.