Compressed H-bonds in solids. Pressure-dependent ferroelectric behavior of KDP/DKDP

Hydrostatic pressure effect on ferroelectric structural phase transition (SPT) in KDP (KH2PO4) and DKDP (KD2PO4) as typical examples of H-bonded solids is considered in the frames of pseudospin Hamiltonian, its parameters Ω(H/D) and Jij(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 Tc of SPT on the base of the calculated values of Ω(H/D) and Jij(H/D). In agreement with the experiments for both techniques Tc 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.