Formation and transformation of alkoxy-derived BaB2O4

Barium metaborate was formed by the simultaneous hydrolysis of barium and boron alkoxides. The dehydrated compound of the as-prepared BaB₂O₄ is termed γ-BaB₂O₄. The IR pattern of γ-BaB₂O₄ was able to be ascribed to a chain anion made up of (-BO₂)⁻ units. The transformation of γ- into β-BaB₂O₄ and of β- into αBaB₂O₄ was observed at 590–650°C and 870–940°C, respectively. From the results of the high-temperature X-ray analysis as well as the DTA data, both transformations were found to be irreversible. Transformation isotherms of γ- into β-BaB₂O₄ were described by the first-order equation and activation energies were determined as 69 kcal/mol and 48 kcal/mol for nucleation process and propagation process, respectively. The kinetics of transformation of β- into α-BaB₂O₄ was best interpreted by the contracting cube equation. Activation energies were 126 kcal/mol and 76 kcal/mol for initial and final stages, respectively.