Growth of Sm1 – ySryF3 – y (0 < y ≤ 0.31) Crystals and Investigation of Their Properties

Abstract

Sm_1 – ySr_yF_3 – y (0 < y ≤ 0.31) crystals have been grown from melt by directional solidification in a fluorinating atmosphere. The crystals have been studied by X-ray diffraction and optical spectroscopy, and their fluorine-ion conductivity σ_dc, density ρ, and refractive indices n_D have been measured. It is established that Sm³⁺ ions are not reduced to Sm²⁺ during crystal growth. The reversible polymorphic α ↔ β-SmF₃ transition does not make it possible to obtain bulk (>1–3 mm³) samples of the tysonite phase (of LaF₃ type) at y < 0.02. The dependences ρ(y) and n_D(y) for the crystals are descending. The dependence σ_dc(y) exhibits nonmonotonic behavior; the maximum σ_dc value (1.6 × 10^–4 S/cm) at 293 K is observed for the Sm_0.98Sr_0.02F_2.98 crystal. At y = 0.31, an eutectic composite 69SmF₃ × 31SrF₂ is formed, whose conductivity is σ_dc = 6 × 10^–8 S/cm, a value smaller than σ_dc for the crystal with y = 0.02 by a factor of ~3 × 10³. The carrier concentration n_mob and its mobility μ_mob have been calculated for Sm_1 – ySr_yF_3 – y (0.02 ≤ y ≤ 0.25) within the hopping conductivity model. For the crystal with the highest conductivity (Sm_0.98Sr_0.02F_2.98), n_mob = 4.0 × 10²⁰ cm^–3 and µ_mob = 2.5 × 10^–6 cm²/(V s) at T = 293 K.