ИСТИНА

The isostructural Bi2WO6 (BW) and Bi2MoO6 (BM) are the simplest members of layered perovskite-related compound family with general formula (Bi2O2)(An-1BnO2n+1) (Aurivillius phases). These compounds are ferroelectrics, oxygen ionics, catalysts. The phase transitions of BW and BM have been studied for more than four decades, but the number of transitions and their detailed nature are not yet fully clear. The sequence of phase transitions in Bi2MoO6 can be represented by the following scheme: g -310ºC-> g''' -604 ºC-> g'' -640-670ºC-> g' The first three phases (g, g''', g'') are orthorhombic, last phase (g') is monoclinic. g and g''' are polar, g'' and g' are nonpolar phases. Next methods were used to identify of these phases: X-ray, DTA, DSC, thermo-Raman spectroscopy. g''- phase was found in the early works by DTA and X-ray methods [1]. As for Bi2WO6, there are disagreements concerning to its polymorphism. Bi2WO6 was found in orthorhombic ferroelectric g-phase at room temperature, which transformed into ferroelectric g'''-phase at 660 ºC. g'''-phase exists up to temperatures near 900-960 ºC. In most of the works made in the present moment and earlier, believe that near 900-960 ºC orthorhombic polar phase g''' transforms to the monoclinic phase g'. This transition is defined as simultaneously ferroelectric and reconstructive. The possibility of such transition to be ferroelectric with change of symmetry B2cb  A2/m is doubtful. In this connection, Rae [2] has assumed that in BW before the monoclinic phase a nonpolar orthorhombic phase Fmmm exists, which is similar to the g''-Bi2MoO6. Any experimental evidences of the existence of g''-phase in BW were lacking. We revealed the existence of nonpolar phase g''-Bi2WO6 between g''' and g' by measuring of dielectric permittivity temperature dependence and DSC [3]. So, the temperature dependence of permittivity showed ferroelectric Lambda-anomaly at 930 ºC, and DSC-event associated with reconstructive phase transition was observed at 960 ºC. This allowed to argue that in Bi2WO6 between g''' and g' phases nonpolar g''- phase exists, as in the case of BM. It should be noted, that in the case of Bi2MoO6 g''-phase limited by two DSC-events associated with the g'''g'' and g''g' transitions, while for BW only one DSC-event, associated with reconstructive g''g' transition, is observed (Figure 1). A small region of existence of the g''-phase in pure BW (about 30 degrees) can be extended with doping. So, in BW doped with niobium (Bi2W0.9Nb0.1O6) the g''-phase exists from 700 ºC to melting point. This makes it possible to investigate the atomic structure of g''-phase of Bi2WO6, which has not been previously studied owing to small range and high temperature of its existence. Experimental detection of g'' - phase in Bi2WO6 leads to the conclusion about the similarity of polymorphism in BW and BM. The scheme of polymorphism in pure Bi2WO6 is presented below: g -660ºC-> g''' -930 ºC-> g'' -960ºC-> g' [1] A. Watanabe, H. Kodama, Polymorphic transformations of Bi2MoO6, J. Solid State Chem., vol. 35, pp. 240-245, (1980). [2] A. Rae, J. Thomson, R.Withers, Structure refinement of commensurately modulated Bismuth tungstate, Bi2WO6, Acta Cryst., vol. B47, pp. 870-881, (1991). [3] V.I. Voronkova, E.P. Kharitonova, O.G. Rudnitskaya, Refinement of Bi2WO6 and Bi2MoO6 polymorphism. J. Alloys and Compounds, vol. 487, pp. 274-279, (2009).