Investigation of growth conditions of yttrium-aluminium garnet crystals with titanium impurity

A study has been made of the conditions leading to brown coloration of yttrium-aluminium garnet (YAG) crystals with a titanium impurity, grown by the method of vertical directed crystallization from a melt. It has been established that, during synthesis of specimens from a mixture of oxides Y2O3, Al2O3 and TiO2 in air, the Ti4+ ions do not form part of the YAG structure but an additional phase, yttrium pyrotitanate (Y2Ti2O7), is formed. The resulting crystals are colorless. During crystallization in an inert or reducing atmosphere, the Ti4+ ions are partially reduced to a state with a lower valency. The titanium ions of the lowest valency form part of the YAG structure and substantially increase the lattice parameter. The intensity of the resulting brown coloration of the crystals increases with the TiO2 concentration in the charge. When the crystals are remelted in air, they lose color and decompose into practically pure YAG and yttrium pyrotitanate. The results of annealing YAG-Ti specimens in an oxygen atmosphere and X-ray irradiation of these crystals are analyzed. Also presented are the results of studying some growth defects occurring during crystallization of YAG-Ti