Abstract
In spite of the simplicity of the Raman spectrum of , controversy remains concerning the identification of the Raman spectrum associated with the pure solid. In particular, the exact frequency of the pentagonal pinch mode is still an open question. In this paper, the change of the room-temperature Raman spectrum of an oxygen-free film as a function of the laser power is reported. As previously reported, two distinct Raman spectra are found: a 1469- Raman spectrum under a low incident laser power (=1 mW) and a 1459- Raman spectrum under a moderate incident laser power (=50 mW). The temperature dependences of these two Raman spectra in a large temperature range 10–430 K, and under low, moderate, and high incident laser power are analyzed. The most consistent explanation of these dependences is a phototransformation of oxygen-free under the laser irradiation featured by a dominant 1459- Raman spectrum at room temperature, the 1469- Raman spectrum being assigned to the pure oxygen-free . Our point of view is confirmed by the observation, after a peculiar thermal treatment, of a stable-in-air form featured by a 1459- room-temperature Raman spectrum. Obviously, this result argues against the interpretation of the 1459- spectrum as that of a pure oxygen-free solid.
- Received 9 May 1995
DOI:https://doi.org/10.1103/PhysRevB.52.14839
©1995 American Physical Society