Raman Scattering Study of the Phase Mode in the Incommensurate Phase of K2SeO4

Raman scattering experiment has been performed with high sensitivity for K 2 SeO 4 over a temperature range from 5 K to 135 K with special emphasis put on the observation of the phase mode in the incommensurate phase. The result has revealed that the phase mode in the commensurate phase becomes to exhibit gradually an over damped spectral profile as the commensurate-incommesurate phase transition temperature ( T c =93 K) is approached from below and that the overdamped mode persists even in the incommensurate phase. An analysis based on a damped oscillator model shows that the frequency ω 0 of the phase mode decreases as T c is approached from below according to ω 0 = A ( T 0 - T ) with T 0 =121 K, while it increases slightly in the incommensurate phase with further increasing temperature from T c . Discussion is presented in comparison with the existing theories.