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Far-infrared reflectivity spectra of bismuth germanium oxide(Bil2GeO20)single crystals are measured from roomtemperature down to 10 K.All the reflectivity spectra are fitted to a complex dielectric function ε([w]) in the factorisedform.Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrumweight below 150 K.The temperature dependent static dielectric constants are obtained from the Lyddane-SachTellerrelation based on the obtained oscillator parameters.The dielectric constants are found to increase upon decreasingtemperature.which is attributed to the charge transfer among the ious in the unit cell with the temperature varying.