Longitudinal and transverse dielectric constants of kdp-type ferro and antiferroelectrics

A review is presented of the longitudinal and transverse dielectric properties of KDP-type crystals. An extended pseudo-spin model of Blinc-de Gennes tunneling model is studied. The model is solved within the molecular field approximation and the four-cluster expansion. A detailed quantitative comparison is presented between the theoretical predictions and the experimental data for KH₂P0₄KD₂PO₄and varioius antiferroelcctric ADP-type crystals. For KD₂P0₄very good quantitative fit is obtained to the measured temperature dependence of the spontaneous polarization P_cand the longitudinal and transverse dielectric constants ε_cand ∊_afor both the paraelectric and the ferroelectric regime with a single set of parameters e₀-92K, ej=907K, y = 37K and T = 0K. For KH₂P0₄quantitative agreement is obtained with the data for e_cand e_ain the paraelectric regime with a single set of parameters ε₀= 92 K, ci = 900 K, y-53 K and T = 300 K. The antiferroelectric version of the extended pseudo-spin model is also capable of explaining consistently the temperature dependence of ε_rand e_afor the antiferroelectric crystals NH₄H₂P0₄ND₄D₂P0₄and NH₄H₂AsD₄Strong support is presented for the assumption that short-range interactions between protons dominate the ferroelectric transition for KDP-type crystals.