Abstract
Hydrostatic pressure effect on ferroelectric structural phase transition (SPT) in KDP (KH 2PO 4) and DKDP (KD 2PO 4) as typical examples of H-bonded solids is considered in the frames of pseudospin Hamiltonian, its parameters Ω(H/D) and J ij (H/D) describe the quantum motion of proton/deuteron along the H/D-bond and pair interaction between these nuclei at different bonds, respectively. These parameters are determined by means of the cluster non-empirical calculations (SCF, MP2-MP4, B3LYP) for the characteristic structural fragments of the compounds of interest. The clusters geometry was taken from the neutron-diffraction data, obtained for pressure values P = 0 and P = 16.5 kbar. The molecular field approximation (MFA) together with more refined Bethe cluster approach (BCA) were applied to study the T c of SPT on the base of the calculated values of Ω(H/D) and J ij (H/D). In agreement with the experiments for both techniques T c decreases with the growth of P due to the proton/deuteron transfer barrier width and height reduction as well as owing to the diminution of the protons/deuterons effective interaction. The related BCA results for KDP and DKDP are discussed in more details.
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