Intermediate-range order in the silicate network glasses NaFexAl1-xSi2O6 (x=0, 0.5, 0.8, 1):: A neutron diffraction and empirical potential structure refinement modeling investigation

The local structural environment, and the spatial distribution of iron and aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 (x = 1, 0.8, 0.5, and 0) were studied by high-resolution neutron diffraction combined with structural modeling using the empirical potential structure refinement code. This work gives evidence of differences in the structural behavior of Al3+ and Fe3+, which are both often considered to act as network formers in charge-balanced compositions. The short-range environment and the structural role of the two cations are not composition dependent; hence, the structure of intermediate glasses can then be seen as a mixture of the structures of the two end members. All Al3+ is four coordinated for a distance d[4](Al3+-O)=1.76 +/- 0.01 angstrom. The high-resolution neutron data allow deciphering between two populations of Fe. The majority of Fe3+ is four coordinated (d[4](Fe3+-O)= 1.87 +/- 0.01 angstrom) while the remaining Fe3+ and all Fe2+ (similar to 12% of total Fe) are five coordinated (d[5](Fe-O)=2.01 +/- 0.01 angstrom). Both AlO4 and FeO4 are randomly distributed and connected with the silicate network in which they share corners with SiO4 tetrahedra, in agreement with a network-forming role of those species. On the contrary FeO5 tends to form clusters and to share edges with each other. Five-coordinated Fe is interpreted as network modifier and it turns out that, even if this coordination number is rare in crystals, it is more common in glasses in which it can have a key role on physical properties.