Short-range order of sodium-zinc, sodium-copper, and sodium-nickel pyrophosphate glasses by diffractometric and spectroscopic techniques

Short-range order has been investigated in three glasses with composition 0.30MeO·0.35Na2O·0.35P2O5 (Me = Ni, Zn, Cu), which are composed of small, ionic units. X-ray diffraction has been used to study the cation coordination; average coordination numbers turned out to be about 6 for Ni2+, 5 for Cu2+, and 4.5 for Zn2+. More detailed information has been obtained for Ni2+ through neutron diffraction with isotopic substitution and for Ni2+ and Cu2+ through analysis of their electronic spectra; distorted octahedral and square-based pyramidal geometries are suggested respectively for Ni2+ and Cu2+. Anionic speciation has been investigated by IR and Raman spectroscopies; the Zn glass has been also examined by 31P MAS-NMR spectroscopy. All three glasses were found to be almost exclusively composed of pyrophosphate units. These observations are in agreement with the Van Wazer's fully ionic modifying oxide model for phosphate glasses.