Multiple-Scattering EXAFS Analysis of Tetraalkylammonium Manganese Oxide Colloids

Abstract

X-ray absorption spectroscopy at the Mn K edge was employed to elucidate the structure of colloidal tetraalkylammonium (TAA) manganese oxides in sols and gels obtained by different preparation and heat treatment procedures. Two series of colloidal TAA MnO x prepared with tetrapropylammonium (TPA) and tetraethylammonium (TEA) cations were studied. Several manganese oxides, birnessite, and feitknechtite were also measured and served as model compounds for structural refinements. Near edge structure (XANES) analysis revealed different average valences of the colloidal systems. As synthesized and heat-treated, TAA colloids exhibited an average valence of 3.6-3.7, whereas gelled TAA colloids showed a lower average valence of ∼3.5. Extended absorption fine structure (EXAFS) analysis was carried out to distances of ∼6.0 Å around the central Mn atom using theoretical backscattering phases and amplitudes calculated from the ab initio FEFF code. All multiple-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 that, except for gelled TAA samples, no significant amount of corner-shared MnO 6 units was detected in the colloidal systems. Theoretical EXAFS phases and amplitudes were derived for a monoclinic birnessite MnO x layer structure consisting of edgeshared MnO 6 octahedra. EXAFS refinements utilizing these phases and amplitudes resulted in good agreement with experimental data. Differences in the refined scattering shell distances between the TPA and TEA series suggest a structure influencing effect of the two ammonium ions. Bond angles between neighboring MnO 6 octahedra were determined from the amplitude dependence of a collinear Mn-Mn-Mn MS path on deviations from 180°("focusing effect"). On the basis of the bond angles, and with use of the average valence to define the distribution of tri-and tetravalent manganese in the MnO x layers, three distinct 2D structures are proposed: one for birnessite, one for TAA sols, and one for TAA gels.

Transformations and Decomposition of MnCO3 at Earth's Lower Mantle Conditions

“…In the spectrum for the second area (show in blue in Figure 5), one can see a broad peak at ∼6559 eV and a shift to higher energy of the Mn K-edge (from 6547 to 6548 eV) which indicate an increase in Mn oxidation state. This spectrum is consistent with the Mn-K edge for Mn 3 O 4 (Ressler et al, 1999;Jiao and Frei, 2010). Finally the spectrum collected on the third area (in red in Figure 5) show an Mn-K edge at 6550 eV and broad peak at 6555 eV which is consistent with Mn 2 O 3 (Nam et al, 2007;Jiao and Frei, 2010).…”

Synthesis, Crystal Structure and Magnetic Properties of Bixbyite-type Vanadium Oxide Nitrides

“…Single-scattering and multiple-scattering paths in the V 2 O 3 model structure were calculated up to 6.0Å with a lower limit of 2.0 % in amplitude with respect to the strongest backscattering path. EXAFS refinements were performed in R space simultaneously to the magnitude and the imaginary part of a Fourier transformed k 3 -weighted and k 1 -weighted experimental χ(k) data set using the standard EXAFS formula [23]. Structural parameters that have been determined by a least-squares refinement of a theoretical XAFS spectrum calculated for a V 2 O 3 model structure to the experimental XAFS spectrum are (i) one overall E 0 shift, (ii) DebyeWaller factors for single-scattering paths, and (iii) distances of single-scattering paths.…”

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

“…Single scattering and multiple scattering paths in the a-MoO 3 model structure were calculated up to 5.0 Å with a lower limit of 2.0% in amplitude with respect to the strongest backscattering path. EXAFS refinements were performed in reciprocal space simultaneously to magnitude and imaginary part of a Fourier-transformed k 3 -weighted and k 1 -weighted experimental v(k) using the standard EXAFS formula [39]. Structural parameters that are determined by a least-squares EXAFS refinement of a MoO 3 model structure to the experimental spectra are (i) an overall shift of E 0 , (ii) Debye-Waller factors for singlescattering paths, and (iii) distances of single-scattering paths.…”