The oxygen environments of divalent cations in phosphate glasses and crystals – the changes with composition and ionic radii

Besides the PO ₄-based networks, the structure of phosphate glasses is determined much by the modifier cations. The consideration is focused on glasses (MeO) _x(P ₂O ₅) _1-x with divalent modifiers with concentrations x = 0.33 and 0.50. The packing densities and Me−O coordination numbers of the glasses and the related crystals are compared. The MeP ₄O ₁₁ crystals and the glasses at x = 0.33 have MeO ₆ octahedra for all Me dimensions from Zn to Ca. At x = 0.50, the structures of crystals and glasses show a large variability with different Me−O coordination numbers of 4, 5, 6, and 7 for the Me = Zn, Mg, Fe, Co, Mn, Cd, and Ca. Several factors beyond the ionic radii influence the MeO _m polyhedra. Heterogeneities of 1 nm dimension known from small-angle X-ray scattering experiments of the Mg(PO ₃) ₂ glass are explained by small clusters of interconnected MgO ₆ units within a matrix of isolated MgO ₅ pairs.