Proton tunnelling in KH2PO4

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Abstract
The spectroscopic and crystallographic evidence for a symmetric (single-minimum) versus a double-minimum type O[sbnd]H[sbnd]O potential in KH2PO4-type crystals is reviewed. The results clearly show that both the protons in KH2PO4 and the deuterons in KD2PO4 move between two off-centre sites and freeze onto one of these two sites below the ferroelectric transition temperature Tc . A self-consistent determination of the proton-tunnelling frequency from the observed soft-mode frequency, and other static and dynamic properties, requires the use of the four-particle cluster model as the short-range correlations of the protons around a given PO4 group cannot be neglected. The pressure and temperature-dependence of the proton-tunnelling frequency can be understood on the basis of some very general properties of the O[sbnd]H[sbnd]O potential.