Cubic KR₃F₁₀ (R = Y, Tb) single crystals have been successfully grown using the Bridgman technique. Growth of crystals of this type is complicated due to the hygroscopicity of potassium fluoride and melt overheating. The solution to the problem of oxygen-incorporated impurities has been demonstrated through the utilization of potassium hydrofluoride as a precursor. In this study, the crystal quality, structure features, and optical, thermal and electrophysical properties of KR₃F₁₀ were examined. Data on the temperature dependences of conductivity properties of KTb₃F₁₀ crystals were obtained for the first time. These crystals indicated thermal conductivity equal to 1.54 ± 0.05 Wm⁻¹K⁻¹ at room temperature caused by strong phonon scattering in the Tb-based crystal lattice. Ionic conductivities of KY₃F₁₀ and KTb₃F₁₀ single crystals were 4.9 × 10⁻⁸ and 1.2 × 10⁻¹⁰ S/cm at 500 K, respectively, and the observed difference was determined by the activation enthalpy of F⁻ ion migration. Comparison of the physical properties of the grown KR₃F₁₀ crystals with the closest crystalline analog from the family of Na_0.5−xR_0.5+xF_2+2x (R = Tb, Y) cubic solid solutions is reported.

ScF₃ optical quality bulk crystals of the ReO₃ structure type (space group $Pm\overline{3}m$ , a = 4.01401(3) Å) have been grown from the melt by Bridgman technique, in fluorinating atmosphere for the first time. Aiming to substantially reduce vaporization losses during the growth process graphite crucibles were designed. The crystal quality, optical, mechanical, thermal and electrophysical properties were studied. Novel ScF₃ crystals refer to the low-refractive-index (n_D = 1.400(1)) optical materials with high transparency in the visible and IR spectral region up to 8.7 µm. The Vickers hardness of ScF₃ (H_V ~ 2.6 GPa) is substantially higher than that of CaF₂ and LaF₃ crystals. ScF₃ crystals possess unique high thermal conductivity (k = 9.6 Wm⁻¹К⁻¹ at 300 K) and low ionic conductivity (σ = 4 × 10⁻⁸ Scm⁻¹ at 673 К) cause to the structural defects in the fluorine sublattice.

A series of nonstoichiometric La_1–yBa_yF_3–y (0 ≤ y ≤ 0.12) single crystals with a tysonite-type structure (sp. gr. P-3c1) was grown from the melt by the directional crystallization method in a fluorinating atmosphere, and some physical properties were characterized. The concentration dependence of electrical conductivity σ_dc(y) La_1–yBa_yF_3–y crystals was studied. The composition of the ionic conductivity maximum for this solid electrolyte was refined. It was confirmed that the maximum conductivity σ_max = 8.5 × 10^–5 S/cm (295 K) was observed at the composition y_max = 0.05 ± 0.01. Analysis of the electrophysical data for the group of tysonite-type solid electrolytes R_1–yM_yF_3–y (M = Ca, Sr, Ba, Eu²⁺ and R = La, Ce, Pr, Nd) showed that the compositions of the maxima of their conductivity were close and amount to y = 0.03−0.05. This fact indicates a weak influence of the size effect (ionic radii R³⁺ and M²⁺) on the value of y_max for R_1–yM_yF_3–y solid electrolytes.

Crystals based on alkaline earth metal difluorides are widely used optical materials. In this study, in order to expand the range of optical matrices, multicomponent Pb_1−x−yCd_xSr_yF₂ (0.27 < x < 0.55, 0.06 < y < 0.18) solid solution crystals with a fluorite structure (sp. gr. Fm-3m) were grown from melt using the vertical directional crystallization technique for the first time. The densities and refractive indices of the grown crystals vary depending on the quantitative content components (x and y) in the ranges of 6.6039(5)–7.5232(5) g/cm³ and 1.6403–1.7084, respectively. The optical transmission and electrochemical impedance spectra were studied. The homogenous composition regions of non-cellular crystallization of this ternary solid solution at a crystallization rate of 6 mm/h and an interface temperature gradient of 80 deg/cm were experimentally determined as 0.30 < x < 0.35, 0 < y < 0.6. These grown crystalline materials may be of interest as high-density highly refractive cubic isomorphic hosts and low-temperature ionic conductors (~2 × 10⁻⁵ S/cm at room temperature) for various applications.

KTb₃F₁₀ crystals have been grown from the melt by the vertical directional crystallization. The incongruent character of melting of this compound is confirmed experimentally. Corrections are introduced into the phase diagram of the KF–TbF₃ system. It is found that the optimal pre-peritectic composition for the KTb₃F₁₀ crystal growth corresponds to a KF content of 27.5 ± 0.5 mol % in melt. The KTb₃F₁₀ composition is a part of a nonstoichiometric phase of variable composition. The cubic (sp. gr. \(Fm\bar {3}m\)) lattice parameters are limited to the range from 11.679(1) to 11.663(1) Å.

The single-phase nanocrystalline R_0.9Sc_0.1F₃ solutions (R = La, Pr) were synthesized by soft chemistry methods as transparent xerogels with the tysonite structure (space group \(P\bar {3}c1\)). The ionic conductivity of ceramic samples prepared from these materials was 4.5 × 10^–4 and 2.1 × 10^–3 S/cm at 773 K for R = La and Pr, respectively. The activation energy of the ion transfer in ceramic samples was 0.43 (R = Pr) and 0.48 eV (R = La) in the high-temperature segment and 0.56 eV (R = Pr) in the low-temperature segment. It was shown that the isovalent replacement of La³⁺ (Pr³⁺) cations in the R_0.9Sc_0.1F₃ tysonite solid solutions with Sc³⁺ leads to 3–4-fold decrease in the conductivity of the ceramic electrolytes.