Fe-O coordination environment in Hedenbergite (FeCaSi ₂O _6+x) glass

The structure of Hedenbergite (FeCaSi₂O_6+x) glass, with x ∼ 0 . 1 , was measured by neutron diffraction with Fe isotope substitution (NDIS). The resulting difference functions provide a direct measurement of the local coordination environment around iron and silicon atoms. An atomistic model generated from classical molecular dynamics (MD) simulations was refined using the reverse Monte Carlo (RMC) method and compared to the results of independent ab initio MD calculations. The glass structure consists of an Si-O backbone of four-fold corner-sharing SiO4 tetrahedra, disrupted by excess oxygen associated with the metal, M² + , oxides predominantly coordinated to non-bridging charged oxygen atoms. These metal cations occupy a broad distribution of disordered coordination environments and not as well-defined rigid structural motifs analogous to crystal structures, as often assumed in x-ray spectroscopic analysis. The local structure of Ca²⁺ ions is dominated by five- and six-fold coordination. In contrast, the Fe²⁺ ions are ∼ 50% four-fold coordinated but with significant five-fold and minor six- and three-fold coordination environments. Fe³⁺ ions have a stabilising influence on the Si-O framework (analogous to Al^3＋), but not as a well-defined tetrahedral network former. Instead, Fe^3＋ adopts higher coordination environments, with shorter Fe-O bond distances compared to Fe^2＋. The higher coordination of Fe^3＋ accommodates excess oxygen, possibly weakening the backbone, and explaining the paradoxical observation that viscosity of iron-bearing silicate melts increases with increasing oxidation.