Deuteron Intrabond Motion and Ferroelectricity in KD2PO4

The Slater theory of the ferroelectric phase transition in K${\mathrm{H}}_2$P${\mathrm{O}}_4$ type crystals as modified by Takagi and extended by Senko is examined again in the light of recent measurements of parameters made on K${\mathrm{D}}_2$P${\mathrm{O}}_4$. Making full use of the measurements, and assigning an appropriate numerical value to one adjustable parameter which is not independently measured, the theory gives a nearly correct description of the shape of the spontaneous polarization curve. Also the value obtained for the Curie constant is probably satisfactory in view of limitations of the model and uncertainty of the high-temperature experimental data. The theory fails, however, in at least one important respect; the transition entropy is overestimated. This and other discrepancies are discussed briefly.