| Achoimre: | Cubic K<i>R</i><sub>3</sub>F<sub>10</sub> (<i>R</i> = 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 K<i>R</i><sub>3</sub>F<sub>10</sub> were examined. Data on the temperature dependences of conductivity properties of KTb<sub>3</sub>F<sub>10</sub> crystals were obtained for the first time. These crystals indicated thermal conductivity equal to 1.54 ± 0.05 Wm<sup>−1</sup>K<sup>−1</sup> at room temperature caused by strong phonon scattering in the Tb-based crystal lattice. Ionic conductivities of KY<sub>3</sub>F<sub>10</sub> and KTb<sub>3</sub>F<sub>10</sub> single crystals were 4.9 × 10<sup>−8</sup> and 1.2 × 10<sup>−10</sup> S/cm at 500 K, respectively, and the observed difference was determined by the activation enthalpy of F<sup>−</sup> ion migration. Comparison of the physical properties of the grown K<i>R</i><sub>3</sub>F<sub>10</sub> crystals with the closest crystalline analog from the family of Na<sub>0.5−x</sub><i>R</i><sub>0.5+x</sub>F<sub>2+2x</sub> (<i>R</i> = Tb, Y) cubic solid solutions is reported.
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