The narrow Raman linewidth of a librational mode of alpha-rhombohedral boron and its anharmonic effects

The 527-cm(-1) Raman band of alpha-boron, which has been recently assigned as a librational mode, exhibits a very narrow linewidth. The mechanism of t his narrow linewidth is studied from the viewpoint of anharmonic effects of phonons. The density of states which is responsible to the decay processes is rather large. Anharmonic force constants of individual bonds are evalua ted from the pressure dependence on the phonon frequencies, and are found t o be not small. Accordingly, by these two factors only, the linewidth could not be as narrow as observed. It is found that the essential cause of the narrow linewidth is its exceptionally high frequency. This makes the therma l factor for the up-conversion processes small even at room temperature.