Exactly Soluble Model of the Ferroelectric Phase Transition in Two Dimensions

The Slater model of the two-dimensional potassium dihydrogen phosphate crystal is solved exactly under the additional assumption that the dipoles are excluded from pointing along one direction of the crystal axis. The Curie temperature $T_c$ is not affected by this additional assumption but the phase change becomes a second-order transition. Complete polarization occurs below $T_c$ with specific heat $\sim {(T-T_c)}^{-\frac{1}{2}}$ near and above the Curie point.