Quantum chemistry application to H‐bonded ferroelectrics via mean field theory involving proton correlation

Abstract

Abstract 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 ferroelectric behavior of the TKHS‐like materials qualitatively well. MFA overestimates significantly the critical temperature 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. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004