Temperature Dependence of the Linewidth of the Eg Modes of the Raman Spectra for NaNO3 Crystal.

Abstract

The first-order Raman spectra of Eg (2) (186cm-1) and Eg (4) (1385cm-1) modes in NaNO3 crystal were measured in the temperature range of 77-473 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analyzed using the phonon dispersion curves calculated on the basis of a modified rigid-ion model. The results showed that the temperature dependence of the linewidth is caused by the cubic anharmonic term in the crystal potential energy for external Eg (2) mode (186cm-1), while it is caused by the cubic anharmonic term and quartic one in the crystal potential energy for internal Eg (4) mode (1385cm-1). This suggested that since a NaNO3 crystal has a very large energy gap between internal modes and external ones with three phonon process, decay channel for internal modes has a stronger tendency to affect on phonon lifetime than that in the case of external modes.