Theoretical and experimental studies of the transverse dielectric properties of KD2PO4

The transverse dielectric constant of K${\mathrm{D}}_2$P${\mathrm{O}}_4$ was measured in the temperature range $79<T<\mathrm{}322\mathrm{}$ °K. A large discontinuity appears at the transition temperature $T_c$ indicating a well-defined first-order transition. A new feature of the data is a dome-shaped figure above $T_c$, resembling the magnetic susceptibility of many antiferromagnetic materials. Extension of a well-known pseudospin model for K${\mathrm{H}}_2$P${\mathrm{O}}_4$ to the transverse direction is achieved by taking into account the direct contribution of the displacements of the protons to the transverse polarization. Calculation of the transverse susceptibility for this model within the four-cluster approximation leads to a new independent determination of the Slater-Takagi energy configuration parameters ${\epsilon }_0$ and $\epsilon$. Excellent fit to the data is achieved for the choices $\frac{{\epsilon }_0}{k_B}=92\mathrm{}$ °K, $\frac{{\epsilon }_1}{k_B}=907\mathrm{}$ °K and indicates that the tunneling integral for K${\mathrm{D}}_2$P${\mathrm{O}}_4$ is negligible. These values are very close to those derived by independent measurements of ultrasonic sould velocity and specific heat. The results of the model are also compared with the available data for the transverse susceptibility of Cs${\mathrm{H}}_2$As${\mathrm{O}}_4$.