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Modified strong dipole-proton coupling model and local properties of KDP-type ferroelectrics

The real nature of the ferroelectric phase transition and the isotope effect on Tc for H→ D exchange in KH2PO4 or KDP-type ferroelectrics has not been fully explained [1]. The proton tunneling model [2], the earliest model used to explain the isotope effect, predicts the displacive type of phase transition, but more recent measurements on crystal lattice dynamics suggest the order-disorder mechanism. On the other side, by applying an Ising-type of the order-disorder model one can not explain the isotope effect and other experiments such as chemical shift measurements of 31P, which indicates that both types of mechanism are involved in phase transitions [3]. In the new strong dipole-proton coupling (SDPC) model, protons adiabatically follow dynamics of PO43--dipoles and the isotopic effect is described without proton tunneling [4]. By involving the change of the PO43--dipole in all directions, an additional improvement of the SDPC model was recently proposed [5]. This modified SDPC (MSDPC) model describes the static dielectric properties of the crystal better than the original SDPC model. By employing the MSDPC model to describe the polarization dynamics of KDP in the paraelectric phase, order-disorder mechanism near Tc and crossover to displacive mechanism at higher temperatures are predicted [6]. Investigations of the local static and dynamic properties of these crystals by neutron scattering and various resonance techniques (NMR, EPR) are useful approaches in the description of the phase transition. The MSDPC model is employed to describe these properties of the KDP. Similarities between local scale dynamics and polarization dynamics are expected. The data calculated from the model can be correlated with NMR and neutron scattering experiments. In the typical EPR experiment, the response of the paramagnetic probe inside the crystal lattice is measured. In the KDP-lattice, SeO43- was used to replace the PO43- unit and the local disturbance produced by the probe can be seen in the form of order-disorder dynamics, slower than the dynamics of the PO43- group. Although several experimental and theoretical investigations of the probe dynamics were carried out, the question remains whether this probe exhibits only pure Arrhenius type temperature dependence [7] or the deviation from such behavior in accord with pseudo-freeze-out model [8]. In the MSDPC model, the probe is treated as a "soft" impurity in the crystal lattice. The simulated dynamics shows that the probe follows nearly Arrhenius behavior and its activation energy depends on temperature. In deuterated KDP this dynamics is slower with higher activation energy, in accord with EPR measurements. [1] R. Blinc, B. Žekš, Ferroelectrics 72, 193 (1987) [2] R. Blinc, J. Phys. Chem. Solids 13, 204 (1960) [3] A. Bussmann-Holder, N. Dalal, R. Fu, R. Migoni, J. Phys.: Condens. Matter 13, L231 (2001) [4] H. Sugimoto, S. Ikeda, Phys. Rev. Lett. 67, 1306 (1991) [5] D. Merunka, B. Rakvin, Phys. Rev. B 61, 11967 (2000) [6] D. Merunka, B. Rakvin, Phys. Rev. B 66, 174101 (2002) [7] B. Rakvin, N. Dalal, Phys. Rev. B 41, 608 (1990) [8] G. M. Ribeiro, L. V. Gonzaga, A. S. Chaves, R. Gazzinelli, R. Blinc, P. Cevc, P. Prelovšek, N. I. Silkin, Phys. Rev. B 25 311 (1982)

KDP-type ferroelectrics; NMR; EPR; paramagnetic probe

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