Chemical durability, thermal stability and spectroscopic studies of the influence of Ni²⁺ ions in oxyfluorophosphate glasses

The role of nickel(II) ions in oxyfluorophosphate glasses of the composition 20ZnO–20CaF₂–(60 − x)P₂O₅:xNiO with 0 ≤ x ≤ 1.0 in steps of 0.2 mol% is investigated using chemical durability, thermal stability, FTIR, magnetic moment and optical absorption. Addition of zinc oxide and subsequent doping with nickel oxide considerably enhanced the chemical durability of the phosphate glasses probably due to the replacement of the easily hydrated P–O–P bonds by mechanically stronger and chemically more stable P–O–Ni bonds. The differential thermal analysis studies indicated that the network of ZnO–CaF₂–P₂O₅ glass is most stable at 1.0 mol% of NiO and the broad glass transition range of this composition. FTIR studies revealed that the network of the glasses is mainly based on phosphate units (Qⁱ) placed in different structural groups along with NiO₄ and NiO₆ units. The deconvolution studies indicated the presence of Ni²⁺ ions in both octahedral and tetrahedral sites and the shifting of Ni²⁺ coordination from octahedral to tetrahedral with the increasing concentration of NiO. The effective magnetic moment along with the crystal field parameters obtained from optical absorption spectral studies indicated the presence of Ni²⁺ ions in both octahedral and tetrahedral sites and confirmed the shifting of Ni²⁺ coordination from octahedral to tetrahedral with the increasing concentration of NiO. The studies of NiO effect on the oxyfluorophosphate glass system has revealed the dual role of Ni²⁺ ions as octahedral and tetrahedral coordination.