Growth of K R 3 F 10 ( R = Tb–Er) Compounds by the Vertical Directional Crystallization Method. III: Melting Character and Nonstoichiometry of Cubic KHo 3 F 10 and KEr 3 F 10 Single Crystals

K R 3 F 10 ( R = Ho, Er; sp. gr. Fm3̅m ) crystals have been grown by the vertical directional crystallization. The incongruent melting of these compounds is experimentally established, and the temperatures of the corresponding thermal effects are determined. Narrow regions of homogeneity are found for the studied crystals; these regions are also characteristic of the entire series of K R 3 F 10 crystals under study. The cubic lattice parameter monotonically decreases along the crystal length and varies in the range of 11.5782(2)–11.5654(5) Å for KHo 3 F 10 and 11.5225(1)–11.5102(4) Å for KEr 3 F 10 . The conditions for growing K R 3 F 10 crystals of optical quality from melt are optimized.