Extended X-ray absorption fine structure investigation of local symmetry changes in Co2+- and Ni2+-exchanged zirconium phosphates

Abstract

The EXAFS spectra of Co2+- and Ni2+-exchanged zirconium phosphates (mainly the α-forms) have been measured using synchrotron radiation. The M—O bond distances obtained are correlated with geometries inferred from electronic spectra. It is confirmed that there are changes in site geometry with degree of loading and calcination temperature. There is also further support for the presence of distorted tetrahedral sites in the anhydrous layered α-forms.For all the materials, a second shell is visible between 2.4 and 2.87 A(non-phase-shift corrected) depending on the particular material; these are ascribed to M⋯P distances.The gross changes in these second-shell distances, the M—O bond distances and the inter-layer distances are combined to suggest sites for the metal ions. As for the Cu2+ analogues, the zirconium phosphate layers do not remain rigid after exchange and calcination, but slide (and, perhaps, twist) to accommodate the CO2+ or Ni2+.