Raman and dielectric spectra of the glass and single crystal of the composition Li2Ge7O15 in the frequency range 3–1000 cm−1: II. The influence of phase separation
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The Raman spectra of Li2O · 7GeO2 glasses, which are heat treated under different conditions at temperatures near the temperature corresponding to the first exothermal peak in the DTA curve, are measured in the frequency range 20–1100 cm−1. A comparative analysis of the measured spectra with the use of the data available in the literature and the X-ray diffraction results demonstrates that the glass undergoes phase separation and that phases of compositions close to GeO2 and Li2O · 4GeO2 precipitate at the initial stage. The ordering of both phases varies significantly within the sample and, on the average, is substantially higher than that in the initial glass. This confirms the known assumptions made by researchers that crystal nuclei with a Li2O · 4GeO2 structure arise during heat treatment of lithium germanate glasses. At the phase separation stage under investigation, the phases are formed as nanoinhomogeneities that only slightly affect the transparency and, in the course of heat treatment, increase in size with a further ordering of the structure.
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