G. M. Bancroft

Al K-edge XANES spectra of aluminosilicate minerals

This is the first of a two-part molecular dynamics (MD) study that examines the effects of temperature, pressure, and composition on the structure and properties of ten compositions in the system NaAISiO.-Si02. Results were obtained for collections of at least 1300 atoms at temperatures between 2500 and 4500 K, pressures of 2-5 GPa, and simulation durations on the order of 0.1 ns. Durations and numbers of particles are both about three times larger than for previous MD simulations on molten aluminosilicates. This study addresses experimental matters, including aspects of simulation methodology that affect the accuracy of atom trajectories (hence computed properties). These include Nt (total number of atoms in the MD box) and spatial resolution achieved in the interparticle force calculation. Static melt structures and their systematic variation with temperature and composition are also explored. In the second part (Stein and Spera, in preparation), the mechanism of diffusion is studied in detail, and MD-computed results for a variety of thermodynamic and transport properties are reported and related explicitly to melt structure. The present study employs a pairwise-additive form of the interatomic potential energy function, using the parameters of Dempsey and Kawamura (1984) with electrostatic interactions computed by the Ewald sum technique. Greater than 90% of Si and AI are fourfold-coordinated by 0 and >90% of 0 is in twofold coordination by the tetrahedral (T) cations, showing little change with composition and temperature. T -T coordination is found to have a more distinct dependence on composition than does T -0 coordination. Structures and properties determined with this set of parameters differ from those computed using the parameters published by Angell et al. (1987), which give a less strongly ordered tetrahedral network and rates of diffusion for network-forming ions that are larger by about an order of magnitude. Results indicate that transport properties computed from time-correlation functions (e.g., diffusivity and ionic conductivity) apparently become asymptotic for system sizes (Nt) greater than about 1000 particles. Relative to these values, properties computed with Nt less than about 400-600 particles are generally overestimated by 10-100% and show increased variance. Truncation of the Ewald sum (limiting the number of reciprocal space vectors) introduces additional variance in computed properties. Melt structure (e.g., nearest-neighbor coordination statistics, static pair-correlation functions, and intertetrahedral bridging angle distributions) are less dependent upon system size than transport properties. The particular form and parameterization ofinterionic potential influence computed properties much more than does system size or spatial resolution in the force calculation. Substitution of (Na + AI) for Si is accompanied by a decrease in the mean intertetrahedral bridging (T -0T) angle related to increasing numbers of AI-O-AI bridges and produces broader and less sharply peaked angle distributions. Radial density functions computed from the MD configurations reveal muted structure beyond the T -T 1 coordination shell in comparison with published XRD analyses, although there is evidence of structure due to T-02 and T-T2 correlation near 5 A, especially in the simulation of molten NaAlSiO.. Structures produced by the MD model reflect the effects of using simplistic potential energy functions, as well as characteristically high experiment temperatures and rapid quench rates required by the practical limitations of the MD method. 0003-004X/95/0506-0417

Sulphur characterization in coal from X-ray absorption near edge spectroscopy

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Silicon K-edge XANES spectra of silicate minerals

Abstract High resolution (≤ 0.2 eV) S L-edge X-ray absorption near edge structure (XANES) spectra of several inorganic and organic sulphur model compounds are presented. The spectra were recorded in both total electron yield (TEY) and fluorescence yield (FY) modes. A qualitative interpretation is used to explain the origin of the fine features in the XANES spectra of certain model compounds. The XANES spectra of several coals such as the Rasa coal (Cretaceous, Yugoslavia), P-803 coal (Cretaceous, Canada) and fossilized Latex (Eocene, Germany) have also been recorded. Using the spectra of model compounds as fingerprints, several organic functional groups were identified in untreated coal. These include alkyl and aryl sulphides, alkyl and aryl disulphides, and heterocyclic sulphurs. Sulphonic acid and sulphate were observed for aerially oxidized coal. The S K-edge of several model compounds and coals were also recorded to compare with the L-edge. Advantages and drawbacks of the L-edge and K-edge techniques are described.

Sulfur K- and L-edge X-ray Absorption Spectroscopy of Sphalerite, Chalcopyrite and Stannite

Silicon K-edge x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR). The spectra are qualitatively interpreted based on MO calculation of the tetrahedral SiO44−cluster. The Si K-edge generally shifts to higher energy with increased polymerization of silicates by about 1.3 eV, but with considerable overlap for silicates of different polymerization types. The substitution of Al for Si shifts the Si K-edge to lower energy. The chemical shift of Si K-edge is also sensitive to cations in more distant atom shells; for example, the Si K-edge shifts to lower energy with the substitution of Al for Mg in octahedral sites. The shifts of the Si K-edge show weak correlation with average Si-O bond distance (dSi-O), Si-O bond valence (sSi-O) and distortion of SiO4 tetrahedra, due to the crystal structure complexity of silicate minerals and multiple factors effecting the x-ray absorption processes.

Coordination of Si in Na2O-SiO2-P2O5 glasses using Si K- and L-edge XANES

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS2) and stannite (Cu2FeSnS4) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe2+ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu+ 3d and Fe3+ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu+ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe2+ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 Å, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.

S K- and L-edge XANES and electronic structure of some copper sulfide minerals

Abstract Si K- and L-edge X-ray absorption near-edge structure (XANES) of SiO2-P2O5 and Na2O-SiO2-P2O5 glasses containing P2O5 above 30 mol% were investigated using synchrotron radiation. Both Si K- and L-edge spectra indicate that Si remains fourfold coordinated ([4]Si) with O in these glasses until the content of P2O5 reaches about 32 mol%, at which sixfold coordinated Si ([6]Si) first appears. The Proportion of [6]Si increases qualitatively with increase in the content of P2O5. However, several P2O5-rich glasses contain [4]Si only, possibly pointing to a dependence of [6]Si content on quench rate. These results are consistent with 29Si MAS NMR spectra for silicate-phosphate glasses of similar composition. To estimate further the relative proportions of [4]Si and [6]Si in these glasses using Si K-edge spectra, model composite materials of a-SiO2, containing [4]Si only, and c-SiP2O7, containing [6]Si only, were used to establish the correlation of area ratio for [6]Si and [4]Si edge features with bulk composition. The regression equation may be used for semiquantitative estimation of relative proportions of [6]Si and [4]Si in glasses and other materials of unknown structure with compositions similar to those of the present glass systems.

Polarized X-ray absorption spectra and electronic structure of molybdenite (2H-MoS2)

The S K and L-edge x-ray absorption near-edge structures (XANES) of low bornite, cubanite, chalcocite, covellite, enargite and tetrahedrite have been measured with synchrotron radiation. The near-edge features are interpreted based on comparison with the S K- and L-edge spectra of chalcopyrite and a MO/energy band structure model. The XANES spectra of these sulfides reflect the DOS of unoccupied S s-, p- and d-like states near and above the Fermi level. In tetrahedral Cu-Fe sulfides, the Fe3+ 3d crystal field band has much more significant DOS of unoccupied S 3p-and 3s-like states than the Cu+ 3d crystal field band. For Cu sulfides, the Cu+ 3d crystal field band has the higher DOS of S 3p- and 3 s-like states in tetrahedral structure than in structures with the triangular CuS3 cluster. The shifts in both S K- and L-edges correlate approximately linearly with the energy gap.

Interpretation of Ni2p XPS spectra of Ni conductors and Ni insulators

Polarized S K- and L-edge, Mo L3- and L2-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS2) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4dz2, and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t2g−/t2g+and eg−/eg+sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4dx2 — y2 and 4dxy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S pz- and px,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 px,y-like state is lower than that of the S 3pz state. Polarized S K-edge XANES also reveal the features of antibonding S pz- and px,y-like states in molybdenite. The feature assigned to the S 3 pz-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 px,y-like states.