Electronic structure and local atomic arrangement of transition metal ions in nanoporous iron-substituted nickel phosphates, VSB-1 and VSB-5

The electronic structure and local atomic arrangement of transition metal ions in nanoporous iron- substituted nickel phosphates VSB-1 and VSB-5 have been investigated using X-ray absorption near-edge structure (XANES) spectroscopy at Fe K- and Ni K-edges. The Fe K-edge XANES study clearly demonstrated that substituted iron ions were stabilized in octahedral nickel sites of nanoporous nickel phosphate lattice. A comparison with several Fe-references revealed that the substituted irons have mixed Fe ²⁺/Fe ³⁺ oxidation state with the average valence of +2.8-3. According to the Ni K-edge XANES analysis, the aliovalent substitution of Ni ²⁺ with Fe ²⁺/Fe ²⁺ induced a slight reduction of divalent nickel ions in VSB-5 to meet a charge balance. On the contrary, Fe substitution for the VSB-1 phase did not cause notable decrease in the oxidation state of nickel ions, which would be related either to the accompanying decrease of pentavalent phosphorus cations or to the increase of oxygen anions. In conclusion, the present findings clearly demonstrated that the nanoporous lattice of nickel phosphate can accommodate effectively iron ions in its octahedral nickel sites.