Studies of Phase Transitions in Order-Disorder Ferroelectrics. II. Calorimetric Investigations of KD2PO4

Abstract

Calorimetric measurements on K${\mathrm{D}}_2$P${\mathrm{O}}_4$ have been conducted, and the transition is found to be first order with an entropy change at $T_c$ of $(0.2407\mathrm{}\pm 0.0016)R$. The entropy change associated with destruction of the polarization from 0°K to $T_c$ is found to be $(0.4234\mathrm{}\pm 0.0071)R$. It is shown that a mean-field theory of the type proposed by Silsbee, Uehling, and Schmidt can give a good account of the heat capacity and polarization to within about 1°K of $T_c$, although the latent heat and entropy are not predicted satisfactorily, presumably because of the breakdown of the mean-field approximation close to the transition. Below $T_c$ the divergence in the heat capacity may be argued to be logarithmic in the region closer than 1° to $T_c$, or to be as $\frac{1}{{(T_c-T)}^{\frac{1}{2}}}$ in the region $T_c-2\mathrm{}°<T<\mathrm{}{T}_c-0.2\mathrm{}°$. Arguments are offered in favor of the logarithmic divergence.