Theory of dielectric susceptibility in an applied electric field near the incommensurate-to-commensurate phase transition in K2SeO4-type crystals

A phenomenological theory of the dielectric susceptibility in the biasing electric field near the incommensurate-to-commensurate improper ferroelectric phase transition is presented. The incommensurate phase is treated in the soliton limit, i.e., as a periodic sequence of domains with opposite polarisation separated by domain walls of negative energies. Experimental results can be explained quantitatively on the assumption that in the AC electric field used for the measurement the number of domain walls remain constant since the processes leading to its change are much slower.