Continuum equations for ferroelectric crystals with molecular groups are deduced from the long-wave limit of the lattice equations discussed in a companion paper. This allows one to compare the resulting equations to those of continuum models such as the micropolar elasticity of molecular crystals and the linearized continuum theory of piezoelectric ferroelectrics. All the macroscopic coefficients are evaluated in terms of microscopic parameters and numerical values are given for ${\mathrm{KNO}}_3$. The results thus obtained ascertain the validity, in the range of long wavelengths, of the previously constructed nonlinear phenomenological theory of deformable ferroelectrics. Moreover, the dynamic dielectric susceptibility is determined and resonances of the susceptibility are exhibited. Lastly, the stability of the continuum approximation is related to the phase transition and the softening of the acoustic wave velocity in the vicinity of the transition temperature is shown.

• Received 12 November 1985

DOI:https://doi.org/10.1103/PhysRevB.33.6320