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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 Tc is not affected by this additional assumption but the phase change becomes a second-order transition. Complete polarization occurs below Tc with specific heat ~(T-Tc)^-1/2 near and above the Curie point.
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F. Y. Wu (1967) studied this question.
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