Growth of ${\text{N}}{{{\text{d}}}_{{{\text{1}}\; - \;y}}}{\text{Eu}}_{y}^{{{\text{2}} + }}{{{\text{F}}}_{{{\text{3}}\; - \;y}}}$ Single Crystals with Tysonite-Type (LaF₃) Structure and Investigation of the Concentration Dependence of Some Their Properties

Abstract

Nd_1 – yEu_yF_3 – y crystals (y is the molar fraction of EuF₂; 0 ≤ y ≤ 0.15) with a tysonite (LaF₃) structure have been grown from melt by the Bridgman technique; their optical transmission spectra and dependences of the lattice parameters, density, refractive index, and fluorine-ion conductivity on the Eu²⁺ content have been investigated. Nd_1 – yEu_yF_3 – y crystals are single-phase; they are crystallized into the trigonal system (sp. gr. $P\bar {3}c1$, Z = 6) at y ≤ 0.12, whereas at y = 0.15 one can observe stabilization of a high-temperature tysonite α-phase (sp. gr. P6₃/mmc, Z = 2). The crystals under study are transparent in the IR range up to 12 µm. The EuF₂ doping into the NdF₃ matrix leads to a monotonic decrease in the density and refractive index. The dependence of the conductivity on the composition σ_dc(y) has a nonmonotonic character. The Nd_0.97Eu_0.03F_2.97 crystal has the maximum σ_dc value at 293 K (2.0 × 10^–4 S/cm). Its charge-carrier concentration is n_mob = 5.8 × 10²⁰ cm^–3 and the carrier mobility is μ_mob = 2.2 × 10^–6 cm²/(V s) (at 293 K). The σ_dc value for Nd_0.97Eu_0.03F_2.97 is smaller (by a factor of 2.5) than that for Ce_0.97Sr_0.03F_2.97, which has the best electrolytic characteristics among R_1 – yM_yF_3 – y solid electrolytes (M = Ca, Sr, or Ba).