Quantum Chemistry Application to H-Bonded Ferroelectrics via Mean Field Theory Involving Proton Correlation

摘要

A problem of entirely theoretical evaluation of ferroelectric characteristics of H-bonded crystals is considered based on the quantum chemical calculations of different levels.This consideration employs the Ising-type pseudospin formalism and treats the"zero-dimensional'TKHS family crystals M_3(H/D)(AO_4)_2 as convenient examples.Ab initio and post-Hartree-Fock schemes are applied to calculate the parameters of pseudospin Hamiltonians.The simple mean(molecular)field approximation(MFA)and Bethe clusters approach(BCA),which partly takes into account proton-proton(deuteron-deuteron)correlations,are compared.It is found that both mean field approximations describe the unusual low-temperature ferroelectic behavior of the TIHS-like materials qualitatively well.MFA overestimates significantly the critical temperatuer T_C of the structural phase transition paraelectric phase-antiferroelectric phase.The use of BCA provides the T_c value much closer to the experimental data.