ИСТИНА

The peculiarities of corundum structure are caused by the method of obtaining. Corundum was synthesized from the hydrargillite (aluminum hydroxide) in SCW: at temperature 400°C and pressures of water vapor 26 MPa. The transformation of hydrargillite into corundum proceeds in supercritical water on the solid-phase mechanism with intermediate, boehmite. The kinetics of corundum formation is described by the Erofeev equation for a case of ramified chain nucleation. The occurrence of corundum starts with formation of thin hexagonal plates. The growth of crystals proceeds on two mechanisms: surface flowing and oriented crystallization. The formation of corundum structure in conditions of quasi-equilibrium with water vapor is accompanied by appearance of oxygen vacancies and hydroxyl groups. During long treatment in SCW the structure of obtained crystals is perfected. The residual hydroxyl groups and oxygen vacancies are removed from crystal. The annealing of corundum at high temperatures also results in removal of residual hydroxyl groups and change of a state of point defects of structure.