METHOD OF ANNEALING CRYSTALS OF GROUP IIA METAL FLUORIDES Russian patent published in 2011 - IPC C30B33/02 C30B29/12 

FIELD: physics.

SUBSTANCE: method involves subjecting a grown and hardened, i.e. correctly annealed crystal, to secondary annealing which is performed by putting the crystal into a graphite mould, the inner volume of which is larger than the crystal on diameter and height, and the space formed between the inner surface of the graphite mould and the surface of the crystal is filled with prepared crumbs of the same material as the crystal. The graphite mould is put into an annealing apparatus which is evacuated to pressure not higher than 5·10^-6 mm Hg and CF₄ gas is then fed into its working space until achieving pressure of 600-780 mm Hg. The annealing apparatus is then heated in phases while regulating temperature rise in the range from room temperature to 600°C, preferably at a rate of 10-20°C/h, from 600 to 900°C preferably at a rate of 5-15°C/h, in the range from 900 to 1200°C preferably at a rate of 15-30°C/h, and then raised at a rate of 30-40°C/h to maximum annealing temperature depending on the specific type of the metal fluoride crystal which is kept 50-300°C lower than the melting point of the material when growing a specific crystal, after which the crystal is kept for 15-30 hours while slowly cooling to 100°C via step-by-step regulation of temperature decrease, followed by inertial cooling to room temperature.

EFFECT: high quality of producing monocrystals of metal fluorides owing to increase in their homogeneity with maximum reduction of defects in grown crystals, which ensures high yield of the material with good optical characteristics, use of a special mode of preparation and carrying out secondary annealing primary grown and hardened crystals of metal fluorides enable to eliminate microinhomogeneities and small-angle off-orientations of crystals.

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