The Narrow Raman Linewidth of a Librational Mode ofα-Rhombohedral Boron and Its Anharmonic Effects

Abstract

The 527-cm^-1 Raman band of α-boron, which has beenrecently assigned as a librational mode, exhibits a very narrow linewidth. Themechanism of this narrow linewidth is studied from the viewpoint ofanharmonic effectsof phonons. The density of states which is responsible to the decay processes israther large. Anharmonic force constants of individual bonds are evaluated fromthe pressure dependence on the phonon frequencies, and are found to be notsmall.Accordingly, by these two factors only, the linewidth could not be as narrow asobserved. It is found that the essential cause of the narrow linewidth is itsexceptionally high frequency. This makes the thermal factor for theup-conversionprocesses small even at room temperature.