Nature and distribution of iron sites in a sodium silicate glass investigated by neutron diffraction and EPSR simulation

TL;DR

This study combines neutron diffraction and EPSR simulations to elucidate the structure of Fe in sodium silicate glass, revealing dominant Fe3+ tetrahedral sites, a minor 5-coordinated Fe population, and their implications for glass properties.

Contribution

It provides the first detailed structural characterization of Fe sites in sodium silicate glass, highlighting the prevalence of tetrahedral Fe3+ and the existence of 5-coordinated Fe, differing from crystalline counterparts.

Findings

Majority of Fe is Fe3+ in tetrahedral coordination.

Presence of 5-coordinated Fe with average Fe-O distance of 2.01 Å.

Fe sites influence physical properties like elastic modulus.

Abstract

The short and medium range structure of a NaFeSi2O6 (NFS) glass has been investigated by high-resolution neutron diffraction with Fe isotopic substitution, combined with Empirical Potential Structure Refinement (EPSR) simulations. The majority (~60%) of Fe is 4-coordinated ([4]Fe) and corresponds only to ferric iron, Fe3+, with a distance d[4]Fe3+-O=1.87$\pm$0.01{\AA} . This is at variance with the 3D-structure predicted by glass stoichiometry. The existence of a majority of [4]Fe3+ sites illustrates a glass structure that differs from the structure of crystalline NaFeSi2O6, which contains only octahedral Fe3+. The EPSR modeling of glass structure shows that [4]Fe3+ is randomly distributed in the silicate network and shares corner with silicate tetrahedra. The existence of a majority of [4]Fe3+ sites differs from the structure of the corresponding crystalline phase, which contains only octahedral Fe3+. The network-forming behavior of [4]Fe3+, coupled with the presence of Na+ ions acting as charge-compensators, is at the origin of peculiar physical properties of Fe-bearing glasses, such as the increase of the elastic modulus of sodium silicate glasses with increasing Fe-concentration. Our data provide also direct evidence for 5-coordinated Fe, with an average distance d[5]Fe-O=2.01$\pm$0.01{\AA}. This second Fe population concerns both Fe2+ and Fe3+. 5-coordinated Fe atoms tend to segregate by sharing mainly edges. The direct structural evidence of the dual role of ferric iron in NFS glass provides support for understanding the peculiar properties of NFS glass, such as magnetic, optical, electronic or thermodynamic properties.