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Polarization dynamics in KDP and DKDP according to the modified strong dipole-proton coupling model

A ferroelectric phase transition temperature Tc in KH2PO4 (KDP) nearly doubles its value (122 K -> 220 K) when D replaces H. An origin of this isotopic effect and a phase transition mechanism are still unclear. The first model able to explain the isotopic effect, the proton tunneling 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, 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 Langevin equations derived from the modified model are used in numerical calculation of longitudinal and transversal polarization dynamics in KDP and DKDP. In calculated longitudinal polarization fluctuation spectra, beside the expected dipole wave peak, a central peak (CP) appears and increases by reaching Tc from below. Above and near Tc the CP also dominates the spectra. Its origin is the relaxation mode with critical slowing down near Tc, in accordance with the order-disorder mechanism. While DKDP shows clear order-disorder behavior in the whole temperature range up to 460 K, KDP shows crossover from the order-disorder behavior near Tc to the displacive one near 460 K. Together with expected peaks, transversal polarization fluctuation spectra show also a flat low-frequency anomaly for KDP and a CP for DKDP. These results correlate with experimental data. References [1] H. Sugimoto: Phys. Rev. Lett. 67, 1306 (1991). [2] D. Merunka, B. Rakvin: Phys. Rev B 61, 11967 (2000).

KH2PO4; central peak

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