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.

No abstract

On basis of the nonempirical methods (SCF, B3LYP, MP2-MP4) and several cluster models, the ferroelectric KH 2 PO 4 (KDP) and its deuteroanalogue are studied. The tunneling integrals ⍀ and the parameters of the effective coupling of protons/deuterons U,V (the Ising parameters) are calculated for these materials with the 3d network of H/D-bonds. Using the obtained U,V and ⍀ values in the frames of molecular field approximation, it is found that the structural phase (ferroelectric) transition occurs for both crystals within the lowering of temperature. Such low-temperature behavior differs the 3d KDP-family materials from 0d systems, where the low-temperature phase transition takes place only upon deuteration. It is demonstrated that this difference is associated with an abrupt Ising parameters growth for KDP if compared with the nondeuterated 0d materials.

An effect of the AO 4 tetrahedron reorganization vibrational modes on the deuteron tunneling splitting ⌬ D is treated for fully deuterated M 3 D(AO 4 ) 2 materials (M alkali metal; A ϭ S, Se) of the TKHS family using various quantum chemical models and computational techniques. It is found that the reorganization of the nonhydrogen framework of the D(AO 4 ) 2 3Ϫ dimers in a crystal results in the reduction of ⌬ D and favors the transition of a material in the ordered (antiferroelectric) phase. Numerical estimation of this effect with the use of the available diffraction data leads to only minor reduction of ⌬ D . At the same time, substantial pronounced contributions to ⌬ D reduction is due to the corrections of potential energy profiles, calculated by different quantum chemical techniques.

ABSTRACT:The main problems of quantum chemistry of H-bonded ferroelectrics are treated using the zero-dimensional K 3 H(SO 4 ) 2 -like crystals as suitable examples. Various quantum chemical approaches and computational procedures are applied to evaluate the Ising model coupling parameters that determine different thermodynamic and dielectric properties of these materials. The calculated Ising parameters are employed to describe the peculiarities of ferroelectric behavior of the K 3 H(SO 4 ) 2 family crystals in the framework of mean field approximation. The problems related to the Hbond proton (deuteron) tunneling are also discussed.