Temperature-dependent far-infrared properties of Bi₁₂GeO₂₀ single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide (Bi₁₂GeO₂₀) single crystals are measured from room temperature down to 10 K. All the reflectivity spectra are fitted to a complex dielectric function \varepsilon(\omega) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150 K. The temperature-dependent static dielectric constants are obtained from the Lyddane--Sachs--Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.