Raman spectra of synthetic sapphire

We have studied two samples of synthetic sapphire. Sample 1 was used as a w indow in a cryostat and was subjected to liquid nitrogen temperature for a long time (two years). Sample 2 was stored at room temperature. Under ident ical 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 ban d was observed at a wave number of 724.6 cm(-1) which was related to the di stribution of point defects. On the boundary of the sample, just a fluoresc ent background was measured. The measured Raman spectra resemble the spectr a of gamma -Al2O3 phase. On Sample 2, the spectra from the border were simi lar to those from the middle of the sample. The observed results indicate p ossible conversion of alpha -Al2O3 into gamma -Al2O3 due to prolonged expos ure of synthetic sapphire to low temperatures. (C) 2001 Elsevier Science Lt d. All rights reserved.