Evaluation of scanning transmission X-ray microscopy at the Mn L2,3-edges as a potential probe for manganese redox state in natural silicates

Abstract

Determining the Mn valence variation at the nanometer scale will be an important advance in the study of heterogeneous natural silicates. Here, the potential of the scanning transmission X-ray microscopy at the Mn L2,3-edges (640 -655 eV) as a probe for manganese redox state is evaluated. For this purpose, several natural Mn-silicates (rhodonite, ardennite, piemontite, Mn 4+ -silicate, jacobsite), covering several Mn valence, were analysed to identify the spectral parameters most sensitive to the Mn valence, regardless of the coordination environment, the crystal field strength, the nature and the length of the metal-ligand bonds, and the intraatomic Coulomb and spin-orbit interactions. Two suitable spectral empirical calibrations are thus proposed, linking the Mn valence to two peak intensity ratios: one ratio of intensities from two energy points of the L2 peak (at 651.7 and 655.2 eV), and one ratio of intensities from one energy point of the L2 peak (at 655.2 eV) and one of the L3 peak (at 641.6 eV).Thank to them, the first quantitative Mn valence maps are constructed, with a high spatial resolution (< 40 nm pixel size), opening the way to exhaustive crystallochemical studies of silicates containing Mn with different valences.

Iron L3-edge energy shifts for the full range of possible 3d occupations within the same oxidation state of iron halides

“…1. Although the intensity maximum of the L 3 transitions is commonly used as a simple measure of excitation energy shifts, 7 here we use instead the median 5,28 as a robust measure of the final state energy distribution that can also be applied in cases where there is no distinct maximum, as for the L 3 spectrum of [FeX] + . Significant shifts of the median are observed between all neighbors in the series except for [FeCl] + and [FeBr] + , as can be seen in Fig.…”

“…In the latter cases, the variation cannot be traced back to a single origin, as many other effects can obscure the shifts induced by changes in 3d occupation alone 7,36 . Furthermore, the variation of excitation energies within one formal oxidation state is also sizable compared to the shifts between different formal oxidation states that are of the order of 1-2 eV per oxidation state, 5,28,[36][37][38][39] again highlighting that the excitation energy shift must be complemented by additional data in order to reliably extract information on the oxidation state 7,8 . With the rich multiplet structure resolved, which is very sensitive to the iron 3d occupation, the energy-calibrated highresolution spectra of ground state Fe + and [FeX] + ions can serve as benchmarks for high level quantum theory approaches to further improve on simultaneously reproducing both, spectral signatures and total excitation energies of open shell systems 40 .…”

“…7,36 In the latter cases, the variation cannot be traced back to a single origin, as many other effects can obscure the shifts induced by changes in 3d occupation alone. 7,36 Furthermore, the variation of excitation energies within one formal oxidation state is also sizable compared to the shifts between different formal oxidation states that are of the order of 1—2 eV per oxidation state, 5,28,36–39 again highlighting that the excitation energy shift must be complemented by additional data in order to reliably extract information on the oxidation state. 7,8…”

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy

“…It previously has been shown that the excitation energy systematically decreases within one oxidation state when decreasing the average coordination of the metal center. 76 A change in excitation energy of the order of 0.25 eV per change in coordination number has been reported. 76 Since the coordination number of neutral Mn II (acac) 2 and cationic Mn II (acac) 1 + differs by two, we attribute the observed shift to the different coordination of the metal center in both species.…”

“…76 A change in excitation energy of the order of 0.25 eV per change in coordination number has been reported. 76 Since the coordination number of neutral Mn II (acac) 2 and cationic Mn II (acac) 1 + differs by two, we attribute the observed shift to the different coordination of the metal center in both species.…”