Anharmonic Lattice Dynamics in Na

Abstract

The phonon frequencies and lifetimes in Na at $T=5, 90, 160, 293, \mathrm{and} 361°$K have been computed employing both the self-consistent (SC) and perturbation theories of anharmonic-lattice dynamics. To describe Na an effective ion-ion potential derived from the calculations of Geldart et al. which employs the Geldart-Taylor screening function was used. For all the symmetry directions the computed frequencies were in excellent agreement with the observed values at both 90 and 293°K. The predicted lifetimes, however, did not agree so well, suggesting higher-anharmonic damping effects may be important. Comparison of the SC and perturbation results suggest that the frequency shifts are reduced in the SC theory, that the SC theory agrees marginally better with experiment, but that there is little to choose between the two for Na. The agreement with experiment suggests that Na can be well described by an effective two-body potential.