EXAFS Study of Chromium Ions in the Mixed-Solvent System of Formamide and Ammonium Formate

EXAFS (extended X-ray absorption fine structure) spectra of Cr ions exhibiting two oxidation states, Cr(VI) and Cr(III), in the mixed-solvent system of HCONH₂ and HCOONH₄ have been investigated for the Cr K edge by Fourier transform and parameter-fitting methods. In the mixed-solvent system, it was found that Cr(VI) ions form mononuclear CrO₄^2- tetrahedra with Cr-O bond lengths of 1.61 Å and Cr(III) ions form mononuclear [Cr(HCOO)_x(HCONH₂)_y]^3-x (x + y 6; 4 ≤ x ≤ 6) octahedra with Cr-O bond lengths of 2.00 Å. The CrO₄^2- tetrahedra, which exist in the mixed-solvent system at an initial state of dissolution of CrO₃, are transformed into the [Cr(HCOO)_x(HCONH₂)_y]^3-x octahedra by heating reduction.