Structural properties of iron-phosphate glasses: spectroscopic studies and ab initio simulations

Abstract

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 Fe(3+) 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.

Thermal properties of 60P2O5–20Fe2O3–20Al2O3 glass for salt waste immobilization

“…For the higher waste content in the glass, Na ? cations are changing their role from the charge compensators to the glass network modifiers and start breaking the chains build of the structural units proposed in [13]. Thus, up to 30 mol% of Na 2 SO 4 , the melting and the half sphere temperatures are not decreasing, and above this threshold, when the role of sodium is changing, both of these temperatures are decreasing.…”

“…Based on structural model of 60P 2 O 5 -40Fe 2 O 3 , glass is proposed in [13]. The glass is build of small rings of [PO 4 ] and [FeO 4 ] tetrahedral which are joined together in a similar way like in crystalline a-FePO 4 , whose crystal structure is the same as berlinite (AlPO 4 ).…”

“…According to the model, the [FeO 4 [14,16]. Higher concentration of Na 2 O causes that the chains built of the structural units break and separate themselves, which can be observed as a rebuilding of the glass structure [13].…”

“…In the proposed model, Fe 2? ions can behave as typical glass network modifiers joined to non-bridging oxygen atoms and cross-link glass chains and increase the stability of the glass structure [13]. According to the model, the [FeO 4 [14,16].…”

“…It leads to extremely complex crystallography of the iron phosphate system which is composed of over 20 different crystalline phases in which iron can be present in both valence states and coordination numbers to oxygen from 4 to 6. Several structural models of the iron phosphate glasses exist in literature [10][11][12][13].…”