Temperature dependence of the Raman-active lithium modes in LiKSO4and LiNaSO4

External optic vibrational modes originating primarily in the motion of lithium ions are unambiguously identified by Raman scattering from isotopically enriched ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$KS${\mathrm{O}}_4$ and compared with similar modes found in LiNaS${\mathrm{O}}_4$. These modes in both LiKS${\mathrm{O}}_4$ and LiNaS${\mathrm{O}}_4$ appear to be overdamped, broadening with increasing temperature and essentially vanishing at temperatures far below the reported phase-transition temperature. An anomalous maximum in the bandwidths of these modes is observed and discussed.