Growth of KR3F10 (R = Tb–Er) Crystals by the Vertical Directional Crystallization Technique. I: Optimization of the Melt Composition for the Growth of KTb3F10 and Correction of the Phase Diagram of the KF–TbF3 System

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.