Temperature-dependent far-infrared properties of Bi12GeO20 single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide(Bi₁₂GeO₂₀) single crystals are measured from roomtemperature down to 10~K. All the reflectivity spectra are fitted toa complex dielectric function ε(ω) in thefactorised form. Phonon modes at low frequency are found to developupon decreasing temperature and gain considerable spectrum weightbelow 150~K. The temperature-dependent static dielectric constantsare obtained from the Lyddane--Sachs--Teller relation based on theobtained oscillator parameters. The dielectric constants are foundto increase upon decreasing temperature, which is attributed to thecharge transfer among the ions in the unit cell with the temperaturevarying.