Critical anomalies of vibrational line shape at order-disorder phase transitions

We study the critical behavior of the line-shape parameters for stretching excitations in dipolar molecular solids. A theoretical model based on coupled anharmonic oscillators is used, with the assumption of two discrete orientations of the associated dipole moments. At the phase transition from a disordered into an antiferroelectric or ferroelectric dipolar configuration, the line shape is predicted to exhibit inflection-point anomalies with divergent temperature derivatives and universal ratios of the critical amplitudes.