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.

Deuteron tunneling and phase transition in M3D(AO4)2 antiferroelectrics

“…In accordance with (7) The preliminary calculations carried out at the HF/6-311ϩϩ(2d,2p) G level according to the procedure described showed that the final estimations of ⌬ 0 differ only slight (Ͻ10%) from "uncorrected" values of 2⍀, obtained earlier [3]. (Recall that the latter have been calculated using the most realistic potential barrier parameters extracted from numerous HF and post-HF (see above) potential profiles for D(AO 4 ) 2 dimers with both the symmetrical and asymmetrical positions of both deuteron and heavy nuclei.…”

“…If we calculate ⌬ 0 in view of Eq. (11) rather than extract it from [3] (in the form of 2⍀-uncorrected), this seems to result in additional reduction of ⍀ D by Շ5%.…”

“…As earlier, these values of tunneling parameters may be "corrected" by the procedures described in [3], although these procedures require the correction of the corresponding potential energy profile obtained in accordance with Eq. (11).…”

“…The preliminary quantum chemical information about the tunneling parameter ⍀ values in M 3 D(AO 4 ) 2 has been obtained in [2,3] by means of the series of the 1-D potential energy profile calculations corresponding to the deuteron motion. The potential curves have been calculated at the Hartree-Fock (HF) (PP-SBK), B3LYP, MP2, and MP4/ SD(T)Q levels (using GAMESS [6] packages) employing the isolated D(AO 4 ) 2 3Ϫ dimer as the main structural unit of the materials at hand.…”

“…In this article as well as in [2,3] we consider the H-bonded crystals in which the structural phase transitions arise due to the processes of proton or deuteron transfer, similar to chemical reactions of proton or hydrogen atom transfer. The crystalline materials of actively investigated now TKHS family with common composition M 3 (H/D)(AO 4 ) 2 , where M ϭ K, Rb and A ϭ S, Se are chosen as a convenient exam-ple.…”