A Mössbauer study of pentavalent iron in a vanadium(V) oxide matrix
A Mössbauer study of pentavalent iron in a vanadium(V) oxide matrix. Dedushenko S.K., Perfiliev Y.D., Tcheboukov D.E., Pankratov D.A., Kiselev Y.M. //Mendeleev Communications. 1999. V. 9. № 5. P. 211-212
The formation of iron in the 5+ oxidation state was observed in iron-doped vanadium(V) oxide; this state is characterised by a singlet with the isomer shift δ = -0.56 ± 0.01 mm·s-1 relative to α-iron in the Mössbauer spectrum at room temperature.
The preparation and study of the elements in unusual oxidation states are an important branch of present-day inorganic chemistry. These species are of considerable interest because, on the one hand, they are of practical importance creating compounds and materials with new and unusual properties. On the other hand, the data on the physico-chemical properties of these systems expand the experimental basis of empirical laws that remain still quantitatively unsubstantiated.
In this context, a study of iron in high oxidation states is of paramount importance. Indeed, the wine-coloured FeO42– ion has been known since the 19th century. However, the compounds of iron in the oxidation states higher than 4+ are known as only a few of examples to the present day, and the data on their chemical properties are limited.
The compounds of iron in high oxidation states are of particular interest in Mössbauer spectroscopy. Iron seems to be the most convenient element for Mössbauer studies. The correlation between the Mössbauer spectral characteristics and the oxidation state of iron allows us to reveal the potentialities of this technique for studying the electronic and geometric structures of substances.
A study of various compounds doped by different elements is used in modern chemistry and related sciences for determining the positions and properties of dopants. In particular, both simple and sophisticated oxides doped by iron have long been examined by Mössbauer spectroscopy. This approach is promising for the preparation of derivatives of the element in uncommon oxidation states.
We decided on a vanadium(V) oxide matrix for stabilising iron in the unusual 5+ oxidation state for the reasons given below. First, the ionic radii of vanadium are close to the radius of isovalent iron ions. Thus, it is reasonable in terms of crystallography to expect the formation of the Fe5+ ion by isovalent isomorphous substitution of iron for vanadium in V2O5. Second, V2O5 is the highest oxide of vanadium. Consequently, it cannot be a reducing agent for the highly oxidised iron ions. Finally, V2O5 melts at a moderate temperature (674°C) and remains stable even above the melting point. Thus, the problem of uniform iron distribution in the bulk of V2O5 can be solved, for example, by dissolving Fe2O3 in a V2O5 melt.
Here we report the data on the 5+ oxidation state of iron in a vanadium(V) oxide matrix.
If the system is considered on a basis of the crystal structure of vanadium(V) oxide, we can suggest that the formation of pentavalent iron takes place by isovalent isomorphous replacement of vanadium with iron in the V2O5 structure. The coordination polyhedron of vanadium in V2O5 can be considered as a strongly distorted octahedron. This distortion is significant, so that the coordination polyhedron is, in fact, an irregular trigonal bipyramid, which corresponds to the coordination number 5. Further distortion, which appears with the introduction of iron into the system as a result of the difference in the ionic radii of vanadium and iron and of the filling of vacancies in the vanadium oxide structure by Fe2+ and Fe3+ ions, can lead to a decrease in the coordination number.
Thus, we found for the first time that pentavalent iron can be formed in the course of doping vanadium(V) oxide by iron.