Soft mode in the disordered-incommensuratecommensurate phase transitions of thiourea

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Abstract
We present a detailed study of the temperature dependence of the soft lattice vibration of thiourea as observed by infrared reflection and confirmed by Raman scattering in the temperature region T = 20-300°K. The soft infrared active mode passes smoothly through the nearly second order prototype incommensurate transition at ∼200°K but both infrared and Raman active soft modes exhibit a jump at the first order transition (∼170°K) into the commensurate phase in partial agreement with a recently proposed theory. The soft mode strength completely accounts for the temperature dependence of the dielectric constant measured at microwave frequencies 8.2 × 109 Hz and 7.5 × 010 Hz in the commensurate (ferroelectric) and prototype (paraelectric) phases where accurate measurements are possible.