Abstract
Monocrystals of pure and uranium-doped CaF 2 have been grown from the vapour phase in graphite crucibles using a vertical moving gradient method with seed-selection. The growth technique and the properties of the vapour phase grown crystals are described. Evaporation temperatures: 1450–1550 °C, δ T≌ 65 °C, growth temperatures: ca. 1300–1400 °C. Dopant: UF 4. Size of the crystals up to 18 mm in diameter and 50 mm in length. In accordance to the low colour center formation by X-raying the pure CaF 2 crystals contain only a small number of lattice defects. The uranium content and distribution in the doped crystals depends strongly on vaporization temperature and inner shape of the crucibles, the valence of the uranium in the CaF 2(U) crystals is mainly determined by the fluorine vapour pressure in the growth system. Tetravalent U can be incorporated in the presence of PbF 2 and trivalent U under high vacuum.
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