Mn K-edge X-ray Absorption Near-edge Structure Research Articles

Designing novel tunable Mn-based inorganic oxyfluoride pigments

A high tunability of green-blue colors in the manganese-doped oxyfluoride Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga) and anion-deficient Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) is reported, and the chromophores responsible for this intense pigmentation are investigated. The hues exhibited by these materials are quantified via diffuse reflectance UV/Vis spectroscopy and measurement of their direct band gaps via Tauc plot. It is shown that choice of A cation (A = Ca, Sr, Ba) and M cation (M = Al, Ga) for as-synthesized phases Sr2.5A0.5Mn0.1MO4F yield a wide range of green colors (band gap range 2.70–2.96 eV). Treatment of these phases under reducing conditions according to Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) induces anion non-stoichiometry, shifting the observed colors to a wide range of blue/blue-purple hues (band gaps from 3.31 to 3.66 eV), showing potential as tunable inorganic blue pigments. Density field theory (DFT) calculations support the preferential occupation of the smaller 8-coordinate Sr(2) site by the substituted Mn2+ cation. X-ray absorption near-edge structure (XANES) data reveal more subtle nuances in the interplay between formal manganese oxidation state, crystallographic site and observed hue. In general, for as synthesized (green) Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga), the edge position in Mn K-edge XANES is consistent with mixed Mn3+-Mn4+ oxidation state whilst a clear pre-edge structure suggesting that Mn is present on a tetrahedral site. This would suggest that during the reduction step, Mn3+/Mn4+ is reduced to entirely Mn2+ and migrates from the tetrahedral to the Sr(2) lattice site.

Investigations of fine-paste ware production and exchange in maritime Southeast Asia by electron microscopy and synchrotron X-ray absorption

The combination of synchrotron X-ray absorption and electronmicroscopy provided information on the production and exchange of fine-paste ware (FPW), dated to around the 11th to 14th centuries in peninsular Thailand and maritime Southeast Asia. White FPW sherds from Kota Cina in north Sumatra, a well-known trading city in Indonesia, have similar clay composition to those from Kok Moh on Satingphra Spit in peninsular Thailand. Also, homogeneous texture indicating high firing temperatures was found in the samples from Kota Cina as well as in those from Phra Mahathat Temple and Suan Luang Temple in Nakhon Si Thammarat Province in Thailand. However, the split Mn K-edge X-ray absorption near-edge structure (XANES) peaks are exclusively observed in the samples from Nakhon Si Thammarat. The distinctly large compositions of ?-Fe2O3 and carbon, respectively confirmed by Fe K-edge extended X-ray absorption fine structure (EXAFS) and energy dispersive spectroscopy (EDS) spectra, suggest that Nakhon Si Thammarat probably had at least a FPW production site in which the ash glazing process may have been implemented. The evidence of such process is not found in the samples from other sites in maritime Southeast Asia in this analysis.

In Situ Monitoring of Structural and Valence Evolution during Electrochemical Desodiation/Sodiation Process of Na2Fe0.5Mn0.5PO4F

Na2Fe0.5Mn0.5PO4F as a cathode material for Na ion batteries is synthesized by a solid-state method. Synchrotron based in situ X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) techniques are used to study the electronic and crystal structure evolutions of Na2Fe0.5Mn0.5PO4F, complemented by electrochemical analyses during the electrochemical sodiation/desodiation. A reversible capacity of 107 mAh/g can be obtained at 12.4 mA/g with two well-defined voltage plateaus centered at 2.9 and 3.5 V vs. Na/Na+. Clear edge shifts in the Fe and Mn K-edge XANES are observed for Na2Fe0.5Mn0.5PO4F at low current density (i.e., 8 mA/g), relating to the two plateaus in the charging curve associated with the Fe2+/Fe3+ and Mn2+/Mn3+ redox reactions sequentially. Increasing the charge current density to 100 mA/g, similar spectroscopic behavior is observed for Fe K-edge XANES. While, the Mn K-edge shift is smaller compared with this observed at 8 mA/g, indicating the sluggish reaction kinetic of Mn3+/Mn2+ redox couple. Continuous diffraction peaks shifts in in-situ XRD patterns of Na2Fe0.5Mn0.5PO4F are observed throughout the charge-discharge process without emergence of new peaks, indicating of solid-solution processes. Na2Fe0.5Mn0.5PO4F shows good structural reversibility during the electrochemical sodiation/desodiation.

Investigation of microstructures and optical properties in Mn-doped SiC films

Mn-doped SiC films were prepared on Si(100) substrates by RF-magnetron sputtering technique. The microstructures and composition of films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XRD results show that the Mn-doped SiC films form a 3C–SiC crystal structure and some doped Mn atoms occupy the C sites of SiC lattice. The XPS results suggest that there exist SiC, CC and SiMn bonds in the films, but the signal of SiC bond decreases and that of CC bond increases with the increase of Mn concentration. The Mn K-edge XANES and EXAFS measurements show that the majority of Mn atoms form Mn4Si7 secondary phase compounds and the existence of Mn metal and MnO can be safely excluded. Then the photoluminescence (PL) properties of films are observed at room temperature. The origin of 413-nm PL peak is associated with C cluster centers, which obviously correlates with the Mn-doped concentration.

Synchrotron X-Ray Absorption Study of Cu and Mn Doped BiFeO3-BaTiO3 Multiferroic Ceramics

In this work, an experimental X-ray Absorption Spectroscopy (XAS) measurement was employed to determine the local structure and valency of Cu and Mn in BiFeO3-BaTiO3. Synchrotron x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) experiments were performed on Cu-doped and Mn-doped BiFeO3-BaTiO3 samples. The Cu- and Mn-doped BiFeO3-BaTiO3 ceramic samples were used for the XAS experiment. XAS spectra at the Cu and Mn K-edge were recorded in flurorescent mode with 13-element Ge detector. The spectra were collected at ambient temperature with a Ge(111) double crystal monochromator and recorded after performing an energy calibration. The features of the measured Cu and Mn K-edge XANES was consistent with Cu and Mn on the Fe/Ti site and inconsistent with Cu and Mn on other sites. The clear agreement was the strongest evidence of Cu and Mn substituting for B-site in BiFeO3-BaTiO3 materials. In addition, the valency of both Cu and Mn ions in BiFeO3-BaTiO3 materials was also confirmed.

Identification of Mn Site in BaTiO3 by Synchrotron X-Ray Absorption Spectroscopy Measurements

In this work, an X-ray Absorption Spectroscopy (XAS) measurement was employed to determine the local structure of Mn in BaTiO 3 . A combination of Synchrotron X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments was performed on Mn-doped BaTiO 3 samples. The BaTi 1 − x Mn x O 3 powder samples (where x = 0.01–0.2) were used for the XAS experiment. XAS spectra at the Mn K-edge were recorded in transmission mode. The spectra were collected at ambient temperature with a Ge(111) double crystal monochromator and recorded after performing an energy calibration. The features of the measured Mn K-edge XANES and EXAFS were both consistent with Mn on the Ti site and inconsistent with Mn on other sites. In addition, the dielectric and ferroelectric properties results also supported the possible Mn Ti substitution in BaTiO 3 material. The clear agreement between the two measured synchrotron XAS spectra, as well as supporting electrical results, was by far the strongest evidence of Mn substituting for Ti in BaTiO 3 structure.

Determination of Mn Site in BaTiO<sub>3 </sub>by X-Ray Absorption Spectroscopy

In this work, an experimental X-ray Absorption Spectroscopy (XAS) measurement was employed to determine the local structure of Mn in BaTiO3. Synchrotron X-ray absorption near-edge structure (XANES) experiments were performed on Mn-doped BaTiO3 samples. The BaTi0.8Mn0.2O3 powders were used for the XAS experiment. XAS spectra at the Mn K-edge were recorded in transmission mode. The spectra were collected at ambient temperature with a Ge(111) double crystal monochromator and recorded after performing an energy calibration. The features of the measured Mn K-edge XANES were consistent with Mn on the Ti site and inconsistent with Mn on other sites. The clear agreement was the strongest evidence of Mn substituting for Ti in BaTiO3.

Electron- and Hole-Doping Effects in Manganites Studied by X-Ray Absorption Spectroscopy

We investigate the electronic structures of hole-doped, La0.7Ca0.3MnO3, and electron-doped, La0.7Ce0.3MnO3, manganites by x-ray absorption near edge structure (XANES) spectroscopy at the O and Mn K-edges. While, the O K-edge XANES results indicate that Ca and Ce doping induce holes in O 2p derived states, the Mn K-edge XANES do not give any evidence for creation of the Mn4+ (or Mn2+) ions by Ca (or Ce) dopants. Such results further questions the validity of double exchange mechanism in understanding the anomalous properties of manganites.

Mn K-edge XANES and Kbeta XES studies of two Mn-oxo binuclear complexes: investigation of three different oxidation states relevant to the oxygen-evolving complex of photosystem II.

Two structurally homologous Mn compounds in different oxidation states were studied to investigate the relative influence of oxidation state and ligand environment on Mn K-edge X-ray absorption near-edge structure (XANES) and Mn Kbeta X-ray emission spectroscopy (Kbeta XES). The two manganese compounds are the di-mu-oxo compound [L'2Mn(III)O2Mn(IV)L'2](ClO4)3, where L' is 1,10-phenanthroline (Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623-6630) and the linear mono-mu-oxo compound [LMn(III)OMn(III)L](ClO4)2, where L- is the monoanionic N,N-bis(2-pyridylmethyl)-N'-salicylidene-1,2-diaminoethane ligand (Horner, O.; Anxolabéhère-Mallart, E.; Charlot, M. F.; Tchertanov, L.; Guilhem, J.; Mattioli, T. A.; Boussac, A.; Girerd, J.-J. Inorg. Chem. 1999, 38, 1222-1232). Preparative bulk electrolysis in acetonitrile was used to obtain higher oxidation states of the compounds: the Mn(IV)Mn(IV) species for the di-mu-oxo compound and the Mn(III)Mn(IV) and Mn(IV)Mn(IV) species for the mono-mu-oxo compound. IR, UV/vis, EPR, and EXAFS spectra were used to determine the purity and integrity of the various sample solutions. The Mn K-edge XANES spectra shift to higher energy upon oxidation when the ligand environment remains similar. However, shifts in energy are also observed when only the ligand environment is altered. This is achieved by comparing the di-mu-oxo and linear mono-mu-oxo Mn-Mn moieties in equivalent oxidation states, which represent major structural changes. The magnitude of an energy shift due to major changes in ligand environment can be as large as that of an oxidation-state change. Therefore, care must be exercised when correlating the Mn K-edge energies to manganese oxidation states without taking into account the nature of the ligand environment and the overall structure of the compound. In contrast to Mn K-edge XANES, Kbeta XES spectra show less dependence on ligand environment. The Kbeta1,3 peak energies are comparable for the di-mu-oxo and mono-mu-oxo compounds in equivalent oxidation states. The energy shifts observed due to oxidation are also similar for the two different compounds. The study of the different behavior of the XANES pre-edge and main-edge features in conjunction with Kbeta XES provides significant information about the oxidation state and character of the ligand environment of manganese atoms.