Multiple-scattering EXAFS analysis of tetraalkylammonium manganese oxide colloids

X-ray absorption spectroscopy at the Mn K edge was employed to elucidate th e structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures. T wo series of colloidal TAA MnOx prepared with tetrapropylammonium (TPA) and tetraethylammonium (TEA) cations were studied. Several manganese oxides, b irnessite, and feitknechtite were also measured and served as model compoun ds for structural refinements. Near edge structure (XANES) analysis reveale d different average valences of the colloidal systems. As synthesized and h eat-treated, TAA colloids exhibited an average valence of 3.6-3.7, whereas gelled TAA colloids showed a lower average valence of similar to 3.5. Exten ded absorption fine structure (EXAFS) analysis was carried out to distances of -6.0 Angstrom around the central Mn atom using theoretical backscatteri ng phases and amplitudes calculated from the ab initio FEFF code. All multi ple-scattering (MS) paths with a weight of 2% and more with respect to the main Fourier transform peak were included in the refinement. It is found th at, except for gelled TAA samples, no significant amount of corner-shared M nO6 units was detected in the colloidal systems. Theoretical EXAFS phases a nd amplitudes were derived for a monoclinic birnessite MnOx layer structure consisting of edge-shared MnO6 octahedra. EXAFS refinements utilizing thes e phases and amplitudes resulted in good agreement with experimental data. Differences in the refined scattering shell distances between the TPA and T EA series suggest a structure influencing effect of the two ammonium ions. Bond angles between neighboring MnO6 octahedra were determined from the amp litude dependence of a collinear Mn-Mn-Mn MS path on deviations from 180 de grees ("focusing effect"). On the basis of the bond angles, and with use of the average valence to define the distribution of tri- and tetravalent man ganese in the MnOx layers, three distinct 2D structures are proposed: one f or birnessite, one for TAA sols, and one for TAA gels.