Structure of iron phosphate glasses modified by alkali and alkaline earth additions: neutron and x-ray diffraction studies

The structure of iron phosphate glasses modified by additions of K2O and BaO, with nominal molar compositions [(1 − x)(0.6P2O5–0.4Fe2O3)]xMeyO, where x = 0–0.4 in increments of 0.1; Me=K or Ba; and y = 1 or 2, has been investigated using neutron diffraction and x-ray diffraction techniques. Fitted coordination numbers for P–O and Fe–O showed a notable change in the P–ONBO and P–OBO contributions at greater than 20 mol% modifier addition, with barium producing a markedly larger increase in P–ONBO contribution than potassium. Fitting of TN(r) and TX(r) provided average nBaO = 9 and nKO = 6. Iron occurs in a range of coordination sites in these glasses: [6]Fe2+, [4]Fe3+, [5]Fe3+ and [6]Fe3+. The partitioning between these sites gives average nFeO = 5.2–5.8, with barium-doped glasses exhibiting higher average nFeO than potassium-doped glasses. The stronger depolymerizing effect of Ba2+ than K+ on the phosphate network, coupled with its greater ionic charge and higher Me–O, Fe–O and O⋯O coordination numbers, explain previously observed divergences in physical properties between the barium-doped and the potassium-doped glasses.