Chemical Information in the L₃ X-ray Absorption Spectra of Molybdenum Compounds by High-Energy-Resolution Detection and Density Functional Theory

Abstract

X-ray spectroscopy using high-energy-resolution fluorescence detection (HERFD) has critically increased the information content in X-ray spectra. We extend this technique to the tender X-ray range and present a study at the L3-edge of molybdenum. We show how information on the oxidation state, phase composition, and local environment in molybdenum-based compounds can be obtained by analyzing the HERFD L3 X-ray absorption near-edge structure (XANES). We demonstrate that the chemical shift of the L3-edge HERFD spectra follows a parabolic dependence on the oxidation state and show that a qualitative analysis of high-resolution spectra can help to estimate parameters such as distortion of a ligand environment and radial order of atoms around the absorber. In certain cases, the spectra allow disentangling the contributions from bond lengths and angles to the distortion of the ligand polyhedron. Comparison of the high-resolution spectra with theoretical simulations shows that the single-electron approximation is able to reproduce the spectral shape. The results of this work may be useful in every branch of physics, inorganic and organometallic chemistry, catalysis, materials science, biochemistry, and mineralogy where observed changes in performance or chemical properties of Mo-based compounds, accompanied by small changes in spectral shape, are to be related to the details of electronic structure and local atomic environment.

LiMo8O10: Polar Crystal Structure with Infinite Edge-Sharing Molybdenum Octahedra

“…Figure shows XANES measurements of the Mo-L 3 edges of LiMo 8 O 10 and a series of oxide standards with differing valances/configurations: MoO 3 (Mo 6+ , d 0 ), SrFe 0.5 Mo 0.5 O 3 (Mo 5+ , d 1 ), Sm 2 Mo 2 O 7 (Mo 4+ , d 2 ), MoO 2 (Mo 4+ , d 2 ), and elemental Mo (Mo 0 , d 5 ). The intense near-edge “white line” (WL) features in the spectra are due to dipole transitions from the 2p core level into empty 4d final states. − The characteristic chemical shift of the absorption edge to higher energy with increasing valence can be seen in Figure by the systematic shift to the higher energy of the centrum of the WL feature with increasing Mo-valence. − The loss of screening (an increase of binding energy) with increasing valence is typically invoked for such chemical shift trends . The strong splitting of the oxide spectra due to ligand field and band structure effects can complicate the evaluation of the WL chemical shift. − The first moment of the near edge, E M , as defined in eq , has been used to estimate the relative chemical shifts of XANES spectra with strong spectral features. , In applying this method to estimate the relative WL chemical shifts of the spectra shown in Figure , the low energy integration limit, E L = 2516 eV (well below the edge) was used.…”

“…The intense near-edge “white line” (WL) features in the spectra are due to dipole transitions from the 2p core level into empty 4d final states. − The characteristic chemical shift of the absorption edge to higher energy with increasing valence can be seen in Figure by the systematic shift to the higher energy of the centrum of the WL feature with increasing Mo-valence. − The loss of screening (an increase of binding energy) with increasing valence is typically invoked for such chemical shift trends . The strong splitting of the oxide spectra due to ligand field and band structure effects can complicate the evaluation of the WL chemical shift. − The first moment of the near edge, E M , as defined in eq , has been used to estimate the relative chemical shifts of XANES spectra with strong spectral features. , In applying this method to estimate the relative WL chemical shifts of the spectra shown in Figure , the low energy integration limit, E L = 2516 eV (well below the edge) was used. Following the analysis performed in the Re-containing compounds, the high energy limit was chosen as the energy where the postedge absorption coefficient fell below the absorption coefficient of μ = 1.5 (Figure ).…”

“…First, the d-count dominance in the chemical shift has been tacitly assumed because the 4d 5 configuration is common to both Mo 0 (4d 5 5s 1 ) and Mo + (4d 5 5s 0 ). This assumption is partially justified by the quadratic chemical shift versus valence relation found in some theoretical and experimental treatments of the Mo-shift, which flattens in the sub-Mo 2+ range . Second, due to the shortage of Mo 2+ and Mo + standards, the formal LiMo 8 O 10 configuration of Mo 2.375+ (4d 3.625 ) was included in the linear fit.…”

“…This assumption is partially justified by the quadratic chemical shift versus valence relation found in some theoretical and experimental treatments of the Mo-shift, which flattens in the sub-Mo 2+ range. 31 Second, due to the shortage of Mo 2+ and Mo + standards, the formal LiMo 8 O 10 configuration of Mo 2.375+ (4d 3.625 ) was included in the linear fit. With these caveats, the observed chemical shift of LiMo 8 O 10 can be viewed to be consistent with its formally expected valence/configuration.…”

Challenges and Opportunities for Applications of Advanced X-ray Spectroscopy in Catalysis Research

“…In 1991, Hämälainen et al demonstrated the dramatic sharpening of HERFD XAS technique on the L 3 edge of dysprosium, laying the foundation for the increasingly popular HERFD XAS approach. Today, the technique is being widely applied across the periodic table for a variety of elements, including the first, ,− second, − and third − row transition metals, main group elements, − halogens, lanthanides, − and actinides. − …”