Crystal growth and thermodynamic investigation of bismuth germanate with sillenite structure

Abstract

Bismuth germanate (Bi₁₂GeO₂₀) with the sillenite structure was grown by a low-temperature-gradient Czochralski technique. Single crystals of Bi₁₂GeO₂₀ are promising for use in piezoelectrics, photoconductors, spatio-temporal light modulators, etc. For the first time the standard formation enthalpy, stabilization enthalpy and lattice enthalpy for Bi₁₂GeO₂₀ single crystals were determined using reaction calorimetry. The original thermodynamic cycle was developed to determine the standard formation enthalpy, and the heat capacity was measured by differential scanning calorimetry in the temperature range of 320–1000 K. It was shown that single crystals of Bi₁₂GeO₂₀ appear suitable for applications from thermodynamic point of view.