Studies of Phase Transitions in Order-Disorder Ferroelectrics. III. The Phase Transition in KH2PO4and a Comparison with KD2PO4

Abstract

Calorimetric, dielectric, and electrocaloric measurements have been performed on single-crystal samples of K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ in the neighborhood of the ferroelectric phase transition, and show that the transition is of first order, with a discontinuous entropy change of $({{0.0456}_{\ensuremath{-}0.0059}}^{+0.0045})R$ and a change in the dielectric constant by a factor of 2. Calorimetric measurements in applied electric fields were also carried out, and show that the effect of an applied field is to shift the transition to higher temperatures and to broaden it. Calorimetric measurements on K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ are reported which can be combined with previously reported results to give the heat capacity for this material in the range 80-280\ifmmode^\circ\else\textdegree\fi{}K. Comparison of the heat capacity of K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ and its deuterated isomorph allows estimates of the lattice heat capacity to be made. From this estimate, the entropy change associated with the ferroelectric transition is found to be $(0.400\ifmmode\pm\else\textpm\fi{}0.036)R$ for K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ and $(0.457\ifmmode\pm\else\textpm\fi{}0.034)R$ for K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$. A significant feature of the comparison is the lack of evidence for the heat-capacity contribution above the transition predicted by theories of the Slater type. Comparison of the calorimetric data with the modified Slater theory due to Silsbee, Uehling, and Schmidt shows that this theory describes K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ better than it does K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$. The differences between the theory and experiment are of the type predicted by the modifications of the theory which include tunneling motion of the protons.