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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.

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Previously the non-cubic β-pyrochlore polycrystalline compounds Rb 0.95 Nb x Mo 2-x O 6.475-0.5x (x = 1.31-1.663) have been obtained in the ternary system Rb 2 O-Nb 2 O 5 -2MoO 3 . However, using special synthesis condition leads to the formation of cubic Fd-3m β-pyrochlore Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 with small size particles and surface area of 15 m 2 /g. The Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 powder has been investigated by X-ray powder diffraction analysis and scanning electron microscopy with X-ray microanalysis. The crystal structure refinement has been performed using the Rietveld method. The electronic structure and optical band gap of Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 have been studied in comparison to similar non-cubic composition Rb 0.95 Nb 1.625 Mo 0.375 O 5.66 . The photocatalytic properties of Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 have been investigated on the example for dyes solution oxidation under UV irradiation. The dependence on the photocatalytic properties of dye type and catalyst mass has been studied. It has been observed that the main oxidative species for dye oxidation is hydroxyl radicals. The phase and composition stability of catalyst powder have been confirmed.

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