Vitrification is the most effective method for the immobilization of hazardous waste by incorporating toxic elements into a glass structure. Iron phosphate glasses are presently being considered as matrices for the storage of radioactive waste, even of those which cannot be vitrified using conventional borosilicate waste glass. In this study, a structural model of 60P₂O₅–40Fe₂O₃ glass is proposed. The model is based on the crystal structure of FePO₄ which is composed of [FeO₄][PO₄] tetrahedral rings. The rings are optimized using the DFT method and the obtained theoretical FTIR and Raman spectra are being compared with their experimental counterparts. Moreover, the proposed model is in very good agreement with X-ray absorption fine structure spectroscopy (XANES/EXAFS) and Mössbauer spectroscopy measurements. According to the calculations the Fe³⁺ is in tetrahedral and five-fold coordination. The maximal predicted load of waste constituents into the glass without rebuilding of the structure is 30 mol%. Below this content, waste constituents balance the charge of [FeO₄]⁻ tetrahedra which leads to their strong bonding to the glass resulting in an increase of the chemical durability, transformation and melting temperatures and density.

In this work the synthesis of hercynite from Fe₂O₃ and Al₂O₃ powders was carried out by arc-melting method under the protective argon atmosphere. The obtained material was characterized with the use of powder X-ray diffractometry (XRD) and Mössbauer spectroscopy (MS). A Mössbauer effect in hercynite obtained by the arc-melting method indicated the cations distribution in the spinel structure among the tetrahedral and octahedral interstices. The presence of Fe²⁺ ions was detected in both tetrahedral and octahedral sites while Fe³⁺ ions occupied only the octahedral interstices. The approximate formula of the obtained iron-aluminate spinel was as follows (Fe²⁺ _0.77Al³⁺ _0.23) (Fe³⁺ _0.07Fe²⁺ _0.05Al_0.88)₂O₄.

The Fe–Al end-member spinel hercynite was prepared by the arc plasma synthesis (APS) technique and subjected to annealing at 1273 K for 1, 24 and 36 h in air atmosphere. The X-ray diffraction (XRD) analyses conducted before and after the thermal treatment revealed an evident alteration of the original hercynite. About half of the original spinel is preserved after the longest annealing experiment (36 h), the reminder of the Fe–Al spinel underwent decomposition into iron and aluminum oxide phases, among which non-stoichiometric magnetite and corundum were the most important. Mössbauer spectroscopy revealed that the initial hercynite was a normal spinel with a slight inversion degree of 11%, which was increased to 34% after the 36-h annealing process. An overlapping of reflections for spinel-like phases γFe₂ O₃ and γAl₂ O₃ with Fe–Al spinel impeded their detection by XRD. Mössbauer spectroscopy (MS) confirmed the presence of magnetite in the annealed samples. The SEM-EPMA observations of the oxidized surfaces of hercynite showed well-developed, scalenohedral crystals with different Fe/O ratios, and well defined compositions corresponding to Al-bearing Fe₂ O₃ as measured by WDS spectroscopy. The results of XRD, MS and SEM-EPMA permit to establish a general oxidation mechanism of hercynite decomposition.