Spectroscopic studies of ferroelectricity in thiourea

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Abstract
The whole reflectivity spectrum of a single crystal of thiourea polarized along the ferroelectric axis [100] has been fitted with a generalized dielectric constant model at different temperatures from 300 K (phase V) to 130 K (phase I). The different experimental results have been used in order to obtain the whole reflectivity spectrum from 1000 Hz to 100cm−1. We so obtain all the dispersion parameters of the soft mode (ωt γt ω1 and γ1) and we prove the existence of two Debye relaxations. The dependence of damping of the soft mode on temperature is explained with the aid of anharmonic cubic potential. With the aid of a generalized Lyddane-Sachs-Teller (LST) relation we show that the 169 K discontinuity of the static dielectric constant ε0 is due to relaxation vanishing and that the peak at 202 K is mainly due to the soft mode contribution.

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