Local structure and vibrational dynamics in NiWO₄

Systematic studies of nickel tungstate thin film, amorphous and polycrystalline powders were performed by X-ray absorption spectroscopy at the Ni K and W L1,3 edges, X-ray diffraction and Raman spectroscopy. We found that in spite of the similarity of the local environment around nickel and tungsten ions in all three materials, there is strong difference in the Ni-O and W-O interactions for thin film/amorphous powder and polycrystalline powder. The nickel-oxygen bonding becomes stronger by going from thin film or amorphous powder to polycrystalline tungstate at the expense of the tungsten-oxygen bonding strength. Besides, in thin film and amorphous NiWO4, nickel and tungsten ions have plane square-like [NiO4]O2 and nearly tetrahedral-like [WO4]O2 coordinations, whereas in polycrystalline NiWO4 both ions have distorted octahedral-like [Ni(W)O4O2] environment. The observed phenomena are related to the grain size effect.