The first-order Raman spectra of
Eg (2) (186cm
-1) and
Eg (4) (1385cm
-1) modes in NaNO
3 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.
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