Temperature dependence of the low-frequency vibrational modes in LiNaSO4and LiKSO4

The Raman spectra of the external modes in LiNaS${\mathrm{O}}_4$ and LiKS${\mathrm{O}}_4$ are measured from 12 to 1000 K. Each crystal has at least one phase transition in this interval. As the temperature increases, one low-frequency mode in each crystal decreases markedly in frequency as the bandwidth rapidly broadens. The relative contributions of overdamping and soft-mode behavior to the observed temperature dependence are calculated. The temperature dependence of the bandwidths are analyzed in terms of current theories. Finally, differences in the dynamics of the constituent ions in the high-temperature phases of the two crystals are discussed.