Critical Phenomena in Order-Disorder Ferroelectrics. I. Calorimetric Studies of KH2PO4

High-resolution measurements of the heat capacity in the immediate neighborhood of the ferroelectric transition in K${\mathrm{H}}_2$P${\mathrm{O}}_4$ are reported. The heat-capacity anomaly is interpreted as a second-order, or nearly second-order, transition. The divergence of the heat capacity may be interpreted as logarithmic, and if so, within experimental error the divergence is symmetric in the sense that the logarithmic slopes are the same above and below the transition. If the divergence is logarithmic, little significant evidence of rounding of the heat-capacity anomaly has been found to within a few millidegrees Kelvin of the heat-capacity peak. It is also possible to interpret the data in terms of a power-law divergence, in which case the heat capacity associated with the transition behaves as ${(T_p-T)}^{-0.50\mathrm{}}$ below the transition and as ${{(T-T_p)}^{-1.0\mathrm{}}}_4$ above the transition, and the anomaly is rounded within about 0.05°K of either side of the heat-capacity maximum. The total transition entropy is found to be $(0.422\mathrm{}\pm 0.014)R$. Dielectric-constant measurements are also reported which may indicate that the temperature of the dielectric anomaly may be about 0.1°K lower than the temperature of the calorimetric anomaly.