Mueller's phenomenological theory of Rochelle salt, NaK${\mathrm{C}}_4$${\mathrm{H}}_4$${\mathrm{O}}_6$·4${\mathrm{H}}_2$O, has been generalized, and the properties of the clamped crystal described. A local field theory of the clamped crystal has been developed on the basis of the structure analysis of Frazer, Danner, and Pepinsky. Special care has been taken to avoid any arbitrary assumptions in the theory. A chart is given which shows conditions for the occurrence of ferroelectricity. It has been shown that the difficulties of Mason and Devonshire's theories are the results of too simplified assumptions on the coefficients of the local field. Various quantities which appear in the theory have been calculated using the fact that the reciprocal susceptibility of the clamped crystal has a single zero value. The calculated coefficients which appear in the Taylor series expansion of Helmholtz free energy agree well with those predicted by the phenomenological theory. An explanation is given for the properties of the mixed crystal series, $\mathrm{Na}\left[{\mathrm{K}}_{\alpha }{\left(\mathrm{N}{\mathrm{H}}_4\right)}_{1-\alpha }\right]{\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{O}}_6·4{\mathrm{H}}_2\mathrm{O}$.

• Received 13 May 1958

DOI:https://doi.org/10.1103/PhysRev.111.1259