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Calculation of the Raman frequency and the damping constant (linewidth) of the stretching modes for the metal-organic compound DMMg close to the paraelectric-ferroelectric transitions

We calculate the Raman frequencies of the two stretching modes of as a function of temperature close to the paraelectric-ferroelectric transition (Tc = 270K) in (CH3)(2)NH2Mg(HCOO)(3) referred as DMMg. By assuming that the Raman frequencies of those two modes which exhibit anomalous behaviour near Tc due to the ordering of the dimethlyammonium cations (DMA(+)) as observed experimentally, are associated with the spontaneous polarization (order parameter), their Raman frequencies and damping constants are predicted at various temperatures. The two models (pseudospin-phonon coupled and energy-fluctuation models) are used for the prediction of the damping constant which also explain the critical behaviour of the observed FWHM data for the stretching modes close to the paraelectric-ferroelectric transition in DMMg.