Quantum field theoretical treatment of a dynamical model of the ferroelectric KDP. II

For pt. I see ibid., vol. 4, 2097 (1971). The results of the previous paper with this title by the authors are applied to the electrical susceptibility and the specific heat of KDP and its deuterated isotope, in their paraelectric phase. It is found that the isotope effect and critical behaviour of these and other properties are explained in a selfconsistent way by our simple model if we allow for the expansion of the short hydrogen bonds on deuteration, an effect which has been discussed in connection with another model of KDP. The theory predicts the logarithmic behaviour of the specific heat above the critical point which has been observed recently by Reese. To explain the ferroelectric transition in full detail appears to require a more sophisticated model which would include short range forces.