Some gas phase processes during the vacuum vertical directed crystallization of a-AI,'*O, are studied in this paper. An effect of these processes on the H, C, N concentrations in the crystals is discussed. The detected pulsations of partial H, and C"0 pressures are a result of bubble destruction at the melt-gas interface. The C' '0 bubbles are formed because of chemical interaction between the melt and carbon-containing phases of container material. Besides the container wall, the H, bubbles can be generated also at the crystallization front supersaturated by H,. The perfect a-Al,O, and a-Al,' '0, crystals grown under the same conditions have noticeable differences of their density, microhardness and lattice parameters. This is explained by an isotopic substitution effect.
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