The structure of CaO–MgO–Al₂O₃–SiO₂ melts and glasses doped with FeO_X–NiO

Neutron and x-ray diffraction measurements have been performed on CaO–MgO–Al₂O₃–SiO₂ (CMAS) glasses doped with NiO–Fe_XO at room temperature, along with x-ray measurements on aerodynamically levitated liquids at ≥2000 K. The disordered structures have been modeled using empirical potential structure refinement to investigate the relation between the aluminosilicate network and the modifying cations. The SiO₄ and AlO₄ tetrahedra are found to have wider Si–O and Al–O bond distance distributions in the glass, and the first Ca–O_n coordination shell is highly distorted, redistributing different populations of long and short bonds between the liquid and the glass. The addition of Fe and Ni at low aluminosilicate content increases the number of free oxygens not bonded to AlO₄ or SiO₄. Mg–O and Fe–O are both found to be predominantly fourfold and fivefold in the liquid and glassy states. Despite these low coordination numbers, their bond angle distributions indicate that they are predominantly in nontetrahedral-type geometries, with ferrous and ferric iron possessing similar coordination environments. The Ca–O and Mg–O average coordination numbers and enthalpies of solution are consistent with their higher reactivity within relatively acidic aluminosilicate melts.