Calculation of lattice dynamics, elastic and dielectric properties of γ-BiB3O6 and δ-BiB3O6

The crystal lattice vibration frequencies, densities of phonon states, elastic moduli, and high-frequency permittivities have been calculated in terms of the density functional theory method for two polymorphs gamma-BiB3O6 and delta-BiB3O6. Based on the calculated densities of phonon states, the temperature dependences of the free energies of two considered bismuth triborate modifications have been constructed, and the temperature of the phase transition between these modifications has been determined (1100 K). The structure of a possible nonpolar praphase of delta-BiB3O6 has been proposed. The polarization of delta-BiB3O6 has been estimated as 131 mu C/cm(2).