Kinetic model and mechanism of Y3Al5O12 formation in hydrothermal and thermovaporous synthesis

Kinetics and mechanism of fine-crystalline yttrium- aluminum garnet (YAG) formation under hydrothermal and thermovaporous treatment were investigated. It was synthesized from the stoichiometric mixture of oxides in the temperature range 200-400degreesC at pressures of water vapor 4.0 - 26 MPa. It was established that in quasi-equilibrium with water vapor conditions the synthesis of YAG proceeds with formation of intermediate substance with Y(OH)(3) structure and amorphous aluminous component. The diffusion of this aluminous component into the Y(OH)3 matrix resulted in the reorganization of oxygen sublattice accompanied with dehydroxylation. The kinetics of YAG formation is described by the equation of solid-phase transformation with the limiting stage of nucleation. Synthesized YAG contains 5 - 7% of water, which corresponds to a hydrogarnet structure. The study of luminescence properties of YAG doped with Nd3+ or Cr3+ ions has allowed to determine the positions of hydroxyl groups and oxygen vacancies in the structure.