Measurements of the specific heat and electric susceptibility of K${\mathrm{H}}_2$As${\mathrm{O}}_4$ and K${\mathrm{D}}_2$As${\mathrm{O}}_4$ have been performed in the region of their respective ferroelectric transitions. Both substances undergo first-order transitions with latent heat of 300 and 570 J/mole, respectively. The entropy associated with the transition is found to be $0.506R$ and $0.500R$, respectively. The Curie constants are found to be 187 and 358 K, respectively. The thermal data are used to deduce the spontaneous polarization. The polarization thus deduced agrees with electrocaloric data for K${\mathrm{H}}_2$As${\mathrm{O}}_4$, the only case where independent polarization data are available. The maximum spontaneous polarizations are found to be 5.25 and 7.20 μC/${\mathrm{cm}}^2$, respectively. It is shown that the theory of Silsbee, Uehling, and Schmidt provides an excellent description of the thermodynamic properties of both materials.

• Received 1 February 1972

DOI:https://doi.org/10.1103/PhysRevB.6.193