Model for a simple order–disorder ferroelectric superlattice

We apply the Ising Model in a Transverse Field (IMTF) to analyse the properties of a potassium dihydrogen phosphate/potassium dideuterium phosphate (KDP/KD∗P) superlattice. This is particularly simple because the main difference in the experimental parameters of the constituents is between the transverse fields; the exchange constants differ only by about 10%. Numerical mean-field theory is applied to calculate the on-site polarization and dielectric susceptibility and their spatial averages 〈〈P〉〉 and 〈〈χ〉〉, which are the macroscopic values for the superlattice. We also calculate the macroscopic pyroelectric coefficient p. Thick-layer superlattices, e.g. 25/25, show two peaks in 〈〈χ〉〉 and p, as if they had two phase transitions, whereas thin-layer superlattices, e.g. 5/5, show one-peak behaviour.