Observation of coupled amplitude modes in the Raman spectrum of incommensurate Na₂CO₃

Temperature-dependent phonons have been observed in the Raman spectrum of incommensurate psi -Na₂CO₃. Two modes are identified as q approximately=0 librational modes of the CO₃^2- groups, which soften at the alpha - beta transition at T_c1=763K. At T_c2=633K a modulation of the structure sets in which is believed to be due to a softening of a librational mode at the incommensurate wavevector q*. Two additional lines that the authors observe in the gamma -phase are assigned to coupled amplitude modes that arise from the coupling of acoustic and librational branches at q=q*.