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Development and application of the modified strong dipole-proton coupling model for KDP-type crystals

Hydrogen-bonded crystals of KH2PO4 or KDP-type have attracted scientific interest for a long time. The origin of a large isotopic effect in Tc when H is replaced by D as well as a phase transition mechanism for ferroelectrics of KDP-type are still unclear. The first model able to explain the isotopic effect, the proton tunnelling model, leads to the displacive mechanism, but recent low-frequency lattice dynamics measurements indicate the order-disorder one. According to the strong dipole-proton coupling model, the difference in calculated ground-state energy between H and D causes the isotopic effect [1]. In order to reproduce a better value of Tc for KD2PO4 (DKDP) and the transversal dielectric constant anomaly, this model was modified by inclusion of dipole components perpendicular to the ferroelectric c-axis [2]. The molecular dynamics simulations based on Langevin equations derived from the modified model were performed to study the longitudinal (&#8220 ; soft mode&#8221 ; ) and transversal polarization dynamics in KDP and DKDP [3]. In addition, local static and dynamic properties of these systems were investigated by applying the same model. Finally, the modified model was adapted to describe antiferroelectrics of KDP-type, i.e. (NH4)H2PO4 or ADP and (ND4)D2PO4 or DADP. The obtained results will be shown and compared with experimental data from IR and Raman spectroscopy, as well as from NMR, EPR and neutron scattering measurements. 1.H. Sugimoto and S. Ikeda, Phys. Rev. Lett. 67, 1306 (1991) 2.D. Merunka and B. Rakvin, Phys. Rev. B 61, 11967 (2000) 3.D. Merunka and B. Rakvin, Phys. Rev. B 66, 174101 (2002)

KDP-type ferroelectrics; isotopic effect; order-disorder phase transition; displacive phase transition

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