  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) is studied by high-resolution neutron diffraction combined with
structural modeling using the Empirical Potential Structure Refinement (EPSR)
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, and hence the structure
of intermediate glasses can then be seen as a mixture of the structures of the
two end-members. All Al3+ is 4-coordinated for a distance
d[4]Al3+-O=1.76$\pm$0.01{\AA}. The high-resolution neutron data allows
deciphering between two populations of Fe. The majority of Fe3+ is
4-coordinated (d[4]Fe3+-O=1.87$\pm$0.01{\AA}) while the remaining Fe3+ and all
Fe2+ (~12% of total Fe) are 5-coordinated (d[5]Fe-O=2.01$\pm$0.01{\AA}). 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. 5-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 they can have a key
role on physical properties.
