Raman spectra of synthetic sapphire

We have studied two samples of synthetic sapphire. Sample 1 was used as a window in a cryostat and was subjected to liquid nitrogen temperature for a long time (two years). Sample 2 was stored at room temperature. Under identical conditions, three types of Raman spectra were measured on Sample 1 in the wave number range 100–800 cm⁻¹. In the spectra from the central part of the sample, seven bands were observed, which are in good agreement with a spectrum of natural sapphire. In the spectra off the center, a subtle band was observed at a wave number of 724.6 cm⁻¹ which was related to the distribution of point defects. On the boundary of the sample, just a fluorescent background was measured. The measured Raman spectra resemble the spectra of γ-Al₂O₃ phase. On Sample 2, the spectra from the border were similar to those from the middle of the sample. The observed results indicate possible conversion of -Al₂O₃ into γ-Al₂O₃ due to prolonged exposure of synthetic sapphire to low temperatures.