Order-Disorder Model Theory for the Ferroelectric Effect in the Dihydrogen Phosphates

A term in polarization energy is added to the Slater-Takagi model of ferroelectricity in K${\mathrm{H}}_2$P${\mathrm{O}}_4$(KD${\mathrm{P}}_{\mathrm{H}}$). This free-energy function relates the shift in Curie point temperature due to deuteration of the compound with the measured change in saturation spontaneous polarization. The dielectric constant ${\epsilon }_c$ obtained by numerical differentiation of the free-energy function has sharp maximum at the Curie temperature. With a change of parameters, the model also reproduces some of the details of antiferroelectric behavior.