NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds

A detailed account of the dependence of X-ray absorption spectra (XAS) on polarisation and light beam directions is given. Anisotropic XAS measured on single crystals, layered compounds, polymers and oriented powders are tabulated. The X-ray absorption cross section by non-magnetic samples is derived, including electric dipole and quadrupole contributions. Orders of magnitude are found for magnetic dipole and electric dipole-octupole terms. Using a spherical tensor expansion of the absorption cross section, simple analytical formulae are given for the angular dependence of the electric dipole and quadrupole contributions to XAS for all crystal point groups. These formulae are valid down to the edge and include many-body effects. The are applied to two experimental examples, the iron K-edge in haematite and the copper K-edge in CuCl42-. For the latter case, spherical tensor components are interpreted in terms of molecular orbitals. The influence of unpolarised and circularly polarised X-rays is discussed, as well as various experimental problems met in angle-resolved XAS. In an appendix, the quadrupole contribution to angular XAS is derived within the multiple-scattering formalism, along with closed expressions for the dipole and quadrupole spherical tensor components.

https://iopscience.iop.org/article/10.1088/0953-8984/2/3/018/pdf

Angular dependence of X-ray absorption spectra

Organic molecules are found everywhere and play an important role in almost all biogeochemical processes occurring on the surface of the Earth (Aiken et al. 1985; Thurman 1985; Schwartzenbach et al. 1993; Senesi and Miano 1994). They are found in soluble and insoluble phases, coatings on mineral and colloidal particles, and in gas phase molecules in soils, sediments and aquatic systems. The activities of macro- and micro-fauna and flora release organic molecules of various sizes and composition. Photochemical reactions in the atmosphere also add certain small chain molecules to the organic carbon content in the environment. Significant compositional variations occur in natural organic molecules, which include small chain carboxylic acids, alcohols and amino acids; and polymeric, polyfunctional and polydisperse macromolecules such as humic and fulvic acids. The behavior of small chain molecules and their influence on different geochemical reactions is well understood. However, understanding of the chemistry of biopolymers and their role in different biogeochemical processes in the environment is poor, which may be attributed to the unavailability of instrumentation to examine the chemistry of natural organic molecules in their pristine state.

Two important properties that dictate the behavior of natural organic molecules and biopolymers in the environment are: functional group chemistry and (macro)molecular structure (Schnitzer 1991). Evaluation of these two properties is complicated by the compositional and structural heterogeneity of the naturally occurring organic molecules, and their ability to form intramolecular and intermolecular H-bonds, which further modify their structure and chemical reactivity. These two properties are interrelated and one influences the other. The chemical composition of natural waters (pH, ionic composition and concentration, redox conditions) and soil and sediment particle surface chemistry (composition, coordination environment, number of reactive groups) also modify their behavior. It is not well understood how each of these environmental variables influences the …

Soft X-ray Spectroscopy and Spectromicroscopy Studies of Organic Molecules in the Environment

The near-edge x-ray absorption fine structure (NEXAFS) at the F 1s threshold has been studied with high-energy resolution for a series of binary fluorides, including KF, TiF4, VF4, VF3, CrF3, CrF2, MnF3, MnF2, FeF3, FeF2, CoF2, NiF2, CuF2, and ZnF2 as well as for SF6 in the gas phase, and for the PF6- and TiF62- molecular anions of the solid compounds KPF6 and K2TiF6. Most of these spectra were measured at the Russian-German beamline at BESSY II, while the spectra of KF and CuF2 were taken under comparable experimental conditions at the D1011 beamline at MAX-lab. The spectra of the solid samples were recorded via the total electron yield. The NEXAFS spectra were taken with the aim to elucidate the role of covalent bonding and its manifestation in x-ray absorption spectra as well as to gain information on the electronic structure of the conduction band along the whole series of 3d transition-metal (TM) fluorides. The spectra of these most ionic compounds of the 3d TM's have been analyzed in a comparative way considering also the F 1s NEXAFS spectrum of the molecular TiF62- anion in solid K2TiF6. In its turn, the latter spectrum has been interpreted by comparing with the F 1s NEXAFS spectrum of the molecular PF6- anion in KPF6 and that of SF6 in the gas phase. In this way, the low-lying empty electronic states of the 3d TM fluorides are shown to be formed by covalent mixing of the TM 3d with the fluorine 2p electronic states. It is further found that the number of low-lying empty electronic states with TM 3d-fluorine 2p hybridized character decreases gradually along the series of 3d TM fluorides, and is essentially zero in the case of ZnF2. (Less)

Low-lying unoccupied electronic states in 3d transition-metal fluorides probed by NEXAFS at the F 1s threshold

The near edge structure up to 30 eV above the ionisation threshold of inner shell edges can be interpreted either as the result of interference of waves scattered from neighbouring atoms or in terms of the local projected conduction band density of states. Methods of calculating near edge structure based on both these viewpoints will be discussed and applications to the near edge structures observed in semiconductors and insulators reviewed. Particular emphasis will be given to those cases where the calculations lead to a simple interpretation of near edge structure. The characteristic inner shell edges observed in electron energy loss spectroscopy have traditionally been used for elemental analysis. Now that spectrometers capable of giving about 1 to 1.5 eV resolution on conventional TEMs and 0.3 to 0.5 eV resolution on cold field emission STEMs have become available the observation of fine structure oscillations has become routine. In particular the large modulations within the first 20 to 30 eV above threshold have become easy to observe and can be detected even when the element is present in low concentrations (below about 10% relative to the majority constituent). It would be advantageous if the fine structures could give some new information on the microstructure of the material, especially if that information could not be obtained in other ways. The materials scientist would like to know whether the near edge structure gives information about elemental charge state, local bonding or the local nature of the conduction band. Alternatively the fine structure might be sensitive to environment or coordination number. Another possibility is that it only reflects interatomic bond distances, but in a more convoluted way than the much weaker extended fine structure which starts about 30eV above threshold. One approach to understanding near edge structure is to collect "fingerprints" from elements in similar environments and try to identify common features (see the article by Brydson in this

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The interpretation of near edge structure

F-K X-ray absorption near edge structure (XANES) spectra of alkaline-earth fluorides have been measured with a high energy resolution using synchrotron radiation. Calculations for the observed XANES spectra have been also carried out by use of the short-range order multiple scattering theory. By comparing the observed spectra with the calculated results, we can estimate the effective anion charge distribution of -0.6, -0.65 and -0.795 for MgF 2 , CaF 2 and SrF 2 , respectively. These calculated F K-edge XANES spectra of alkaline-earth fluorides are more sensitive to the charge distribution than cation K-edge XANES.

F-K XANES Studies of Alkaline-Earth Fluorides

F–K X-ray absorption near edge structures (XANES) of alkali fluorides, Lid, NaF, KF and RbF are measured by a photoelectric yield method. Na–K spectrum of NaF is also measured. The short-range order multiple scattering calculations for each K spectrum of LiF and NaF are performed. From comparison of the observed spectra with the calculated results, the effective cation charges are estimated with q =0.70±0.05 and 0.74±0.04 for LiF and NaF, respectively. These calculated F–K XANES spectra of alkali fluorides are not so sensitive to the charge distributions as cation K XANES. These results are in contrast with the case of alkaline-earth fluorides.

F–K XANES Studies of Alkali Fluorides

In this work, we have analysed the Br K -edge XANES of (CHBr y ) x excited by polarized X-ray in // c and ⊥ c directions for three different doping region. In the low doping region it is proved that mono-anions exist, and the information about the charge distribution on carbon chains around the mono-anions is obtained. The maximal amplitude of the charge density is larger than 0.4. In the intermediate doping region ( y =0.0145), the XANES analyses also confirm the model previously proposed by Oyanagi et al. , in which polyanions Br 3 - and covalently bonded bromine atoms coexist. From the analysis of E // c XANES, it is concluded that Br 3 - has larger polarization approximately shown by Br -0.6 Br -0.2 Br -0.6 than I 3 - by I -0.5 I 0.0 I -0.5 . In the high doping region, there are many possible models more than 10 6 . In this case the detailed analyses would be impossible.

Hiroaki Oizumi

Papers

F-K XANES Studies of Alkaline-Earth Fluorides

F–K XANES Studies of Alkali Fluorides

Short-Range Order Full Multiple Scattering Approach to the Polarized K-Edge XANES of Br Doped in Trans-Polyacetylene

Theoretical studies of polarized xanes of Br doped in trans-polyacetylene