Mössbauer study of oxo derivatives of iron in the Fe2O3-Na2O2 system. Pankratov D.A. //Inorganic Materials. 2014. V. 50. № 1. P. 82-89
Various compositions of oxo derivatives of iron reacting with sodium peroxide have been studied by Mössbauer spectroscopy. We have examined several mathematical models of the measured spectra. The results obtained are inconsistent with hypotheses made previously that such conditions may lead to the formation of compounds of iron in oxidation states above (+6). We demonstrate that a large excess of an alkali peroxide leads, most likely, to the formation of at least two iron(V) derivatives in tetrahedral coordination. In their Mössbauer spectra, they have isomer shifts of −0.45 and −0.51 mm/s and unusually large quadrupole splittings: 1.32 and 1.94 mm/s (at room temperature).
It is common practice to divide the charge states of elements in compounds into lower, medium, and higher oxidation states (OS’s). Elements in the most widespread compounds are thought to be in medium oxidation states. As a rule, they possess high thermodynamic stability under various conditions. The elements in them may exhibit both oxidizing and reducing properties. In compounds of elements in their higher oxidation states, the elements are formally in a high charge state and typically exhibit oxidizing properties. Such compounds usually have low thermodynamic stability or are only stable under certain conditions (temperature, pH, solvent, etc.). Note that the theoretically possible higher oxidation state should be differentiated from the experimentally achievable one. For example, in the case of iron, +2 and +3 are commonly thought to be medium oxidation states. It is known that, in most iron-containing minerals, this element is in these oxidation states. Compounds containing iron atoms in higher oxidation states are referred to as higher oxidation states. In particular, +6 for iron is thought to be the highest experimentally achievable oxidation state. Only a small number of such substances, ferrates(VI), are known. They possess strong oxidizing properties, and their stability regions are limited by low temperature and by either a solid phase or highly alkaline solutions. There are only a few examples of the oxidation states intermediate between (+3) and (+6) for iron. As a rule, they form and exist under very unusual conditions.
Theoretically, the highest achievable oxidation state of iron is thought to be (+8). Moreover, there are unconfirmed reports on the synthesis and characterization of the corresponding iron compounds. In spite of the internal contradictions in describing the properties of materials containing iron in this high charge state, the preparation, stabilization, and practical application of iron compounds in extreme oxidation states have been the subject of patents. Note that Mössbauer spectroscopy has been the only method used to identify the charge state of iron in samples. In particular, in a recent study supposedly confirmed the possibility of formation of iron(VII) and iron(VIII) compounds in the products of spontaneous degradation of iron compounds forming in an excess of sodium peroxide. They arrived at this conclusion by interpreting Mössbauer spectra. Using complex mathematical tools, they separated out several resonance lines from poorly resolved spectra, with isomer shifts in the range –1.58 to ⎯0.73 mm/s, which were assigned to compounds of iron in extreme (and exotic) oxidation states (from +8 to +5.5) using extrapolation of data for known iron oxo compounds. The data presented in that report appear, however, superficial and unsystematic, and obviously require further verification.
Using Mössbauer measurements at different temperatures, we have studied solid solutions of Fe2O3 in sodium peroxide at mole fractions of iron from 91.1×10–3 to 1.4×10–3. We have examined different descriptions of the sum spectrum for samples with a mole fraction of iron from 9.8×10–3 to 1.4×10–3 and showed that models which assume the formation of compounds containing iron in oxidation states above (+6) are inadequate. Solid peroxide fusion cakes contain over 80% Fe(V) compounds, possibly with dioxygen ligands, which have an unusually large quadrupole splitting.