Mössbauer Diagnostics of the Isomorphic Substitution of Iron for Aluminum in Triclinic Iron Vanadate. Pankratov D.A., Yur’ev A.I. //Bulletin of the Russian Academy of Sciences. Physics. 2013. V.77. №6. P.759-764.
Mössbauer spectroscopy and X-ray powder diffractometry (XRD) phase analysis are used to study mixed vanadates with the composition AlxFe3– xV3O12, where x is 0, 0.3, 1.0, 1.5, 2.0, 2.7 and 3.0. Mössbauer spectroscopy is used to study the distribution of trivalent cations over different crystallographic positions. It is shown that the distribution of aluminum atoms differs from those expected from statistic and thermodynamic models.
Materials based on iron and vanadium compounds, which include material doped by transition or nontransition metals, are widely used as catalysts and sensor materials. Apart from solid solutions, two individual compounds form in the iron(III) oxide–vanadium(V) oxide system in particular: Fe2V4O13 and FeVO4. In turn, iron ortovanadate FeVO4 crystallizes depending on the conditions in an orthorhombic or triclinic system. In addition to its use in catalysis, triclinic iron vanadate has recently attracted attention due to its interesting magnetic properties. The type of iron vanadate formed at normal pressures is a multiferroic, and antiferromagnetic ordering is observed in it at temperatures below 22 K.
The special magnetic properties of triclinic iron vanadate are determined by its structural properties. It is known that in the crystal lattice of triclinic iron vanadate FeVO4, both iron and vanadium atoms occupy three nonequal crystallographic positions. The different positions of vanadium and iron atoms determine the different neighboring atoms. If in the case of vanadium changes in the oxygen environment are only minor (the vanadium–oxygen distances change but the tetrahedral motive is preserved), in the case of iron the changes are greater. Two iron atoms are thus located in differently distorted octahedral polyhedrons. The sizes of the voids in oxygen octahedrons differ: for one atom, the iron–oxygen distances can vary from 1.936 to 2.102 Å; for another, they can vary from 1.960 to 2.045 Å. The third iron atom is inside a severely distorted trigonal bipyramid with iron-oxygen distances of 1.869 to 2.019 Å. Three pairs of the described iron-containing polyhedrons are connected in tandem along their edges in S-shaped planar fragments that contain six iron atoms each. In the crystal lattice, the latter are arranged in layers and connected in between by vanadium containing tetrahedrons. Considering the above description of the compound’s structure, the properties of iron vanadate should be described more precisely by the formula Fe3V3O12.
This is confirmed by Mössbauer spectra, in which three doublets are found in different crystallographic positions. The doublet with great quadrupole splitting corresponds to the iron atoms in a trigonal bipyramid, while the two doublets with smaller quadrupole splitting and similar isomeric shifts are associated with the iron atoms in the voids of an octahedron.
This work examines the Mössbauer spectroscopy of products from replacing iron atoms in triclinic vanadate with aluminum atoms in order to find the distribution order of trivalent cations over different crystallographic positions, and to relate the doublet parameters of iron atoms in the indicated positions.
We have considered the possible correlation between changes in the parameters of Mössbauer spectra caused by the aluminum distribution in AlxFe3–xV3O12 triclinic ortovanadates at x = 0, 0.3, 1.0, 1.5, 2.0, 2.7 and 3.0. It was shown that the distribution of aluminum atoms over different crystallographic positions differs from the one expected from statistical and thermodynamic models. In the hexa nuclear clusters that are the basis of ortovanadate, aluminum atoms are predominantly in the second and third positions, avoiding the end position. The doublets observed in the spectra are associated with iron atoms in different crystallographic positions.