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Nuclear quantum effects in K3H(SO4)2-type crystals

The H-bonded crystals of K3H(SO4)2 (KHS) group exhibit strong nuclear quantum effects. Some of the crystals undergo an antiferroelectric phase transition in the deuterated, but not in protonated form, which is the extreme H/D isotope effect. Additionally, the absence of phase transition is usually ascribed to a quantum paraelectric state of the protonated crystal. In the quantum paraelectric state, a phase transition in a crystal is suppressed by strong quantum fluctuations of atoms, such as zero-point motion or tunneling. To elucidate a mechanism of the quantum effects in KHS crystals, it is important to determine a proton potential in the H bond. However, the neutron scattering measurements give controversial results. The neutron diffraction measurements [1] and the low-energy peak in inelastic neutron scattering spectra [2] indicate a double-well potential and a proton-tunneling mechanism of quantum effects. However, the neutron Compton scattering measurement of proton momentum distribution [3] indicates a single-well potential and a different mechanism. The theoretical considerations [4] show that the inconsistency between H-bond potentials derived from the position and momentum distributions of proton can be explained by the assumption that the proton does not reside in a well-defined single-particle potential, but rather in a potential that is strongly affected by a local vibrational mode of heavy atoms. The measured low-energy excitation responsible for the quantum paraelectric state is attributed to this local mode and not to the proton mode. The isotope effect is explained by different effective potentials of the local mode in protonated and deuterated crystals due to the mass effect and the geometric effect on the ground-adiabatic-state energy of proton. [1] N. Onoda-Yamamuro, et al., J. Phys.: Condens. Matter 12, 8559 (2000). [2] F. Fillaux, et al., Chem. Phys. 154, 135 (1991). [3] D. Homouz, et al., Phys. Rev. Lett. 98, 115502 (2007). [4] D. Merunka and B. Rakvin, Phys. Rev. B 79, 132108 (2009).

hydrogen-bonded crystals; nuclear quantum effects

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