Proton crystal model

Static dielectric measurements [8] show that all crystals in the family exhibit a very large quantum effect of isotope replacement H D on the critical temperature. This effect can be exemphfied by the fact that Tc = 122 K in KDP and Tc = 229 K in KD2PO4 or DKDP. KDP exhibits a weak first-order phase transition, whereas the first-order character of phase transition in DKDP is more pronounced. The effect of isotope replacement is also observed for the saturated (near T = 0 K) spontaneous polarization, Pg, which has the value Ps = 5.0 xC cm in KDP and Ps = 6.2 xC cm in DKDP. As can be expected for a ferroelectric phase transition, a decrease in the temperature toward Tc in the PE phase causes a critical increase in longitudinal dielectric constant (along the c-axis) in KDP and DKDP. This increase follows the Curie-Weiss law. Sc = C/(T - Ti), and an isotope effect is observed not only for the Curie-Weiss temperature, Ti Tc, but also for the Curie constant C (C = 3000 K in KDP and C = 4000 K in DKDP). Isotope effects on the quantities Tc, P, and C were successfully explained within the proton-tunneling model as a consequence of different tunneling frequencies of H and D atoms. However, this model can hardly reproduce the Curie-Weiss law for Sc-... 

The soft-mode spectra in the FE phase was investigated within the same study [ 19] and a well-defined peak (S-peak) was found at 150 cm for T Tc as the lowest frequency peak in the spectra. The frequency of S-mode decreases with the increase in pressure, indicating that the S-mode is the soft mode and that the phase transition is of the displacive type, which is in accordance with the proton-tunneling model. Furthermore, Raman scattering experiments on deuterated crystals showed the disappearance of the S-peak in DKDP [20]. Since this phenomenon can also be explained by the protontunneling model, it is taken as another important piece of evidence for this model. 

In order to confirm the proton transfer mechanism proposed previously [160], the results of IQNS on terephthalic acid were reported [164]. The jump distance is calculated to be 0.7 A for the proton transfer model and 2.1 A for the 180° rotation model - the latter process was ruled out on the basis of the experimental IQNS results, leading to the conclusion that the mechanism of the proton dynamics is indeed a double proton exchange. IQNS results for terephthalic acid and acetylene dicarboxylic acid have also been reported [165]. For both samples, the jump distance was found to be less than 1 A. For acetylene dicarboxylic acid, single crystal measurements yielded a jump distance of 0.73 A. The Q-depen-dence was found to be in excellent agreement with the 2-site jump model. From these results, the 180° rotation model can be ruled out in favour of the proton transfer model. 

Figure 4. Vacancy (Ruetschi) model for the crystal structure of y - Mn02. The shaded octahedra represent the / -Mn02 parts of the lattice. The small gray circles represent protons attached to oxygen atoms.

Ralph, J. Landucci, L. L. Nicholson, B. K. Wilkins, A. L. Adducts of anthrahydro-quinone and anthranol with lignin model quinone methides. 4. Proton NMR hindered rotation studies. Correlation between solution conformations and X-ray crystal structure. J. Org. Chem. 1984, 49, 3337-3340. 

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