Temperature dependence of line width of SO42− modes in the Raman spectra of ammonium sulphate

The temperature dependence of the Raman line width of the ν1 and ν2 modes of the SO ion in ammonium sulphate was investigated. A sudden change in the full width at half maximum intensity (FWHM) of both bands is observed at the ferroelectric transition point of the crystal. The results are explained in terms of the coupling between phonons originating from these modes and NH4+ ion pseudo‐spins. The results obtained from pseudospin–phonon coupling formulations agree satisfactorily with the experimental results. The coupling also accounts for the temperature dependence of spontaneous polarization.