Au 5d States Research Articles

Ab initio band structure calculations of the low-temperature phases of Ag 2Se, Ag 2Te and Ag 3AuSe 2

Ab initio band structure calculations were performed for the low-temperature modifications of the silver chalcogenides β-Ag 2Se, β-Ag 2Te and the ternary compound β-Ag 3AuSe 2 by the local spherical wave (LSW) method. Coordinates of the atoms of β-Ag 2Se and β-Ag 3AuSe 2 were obtained from refinements using X-ray powder data. The structures are characterized by three, four and five coordinations of silver by the chalcogen, a linear coordination of gold by Se, and by metal–metal distances only slightly larger than in the metals. The band structure calculations show that β-Ag 3AuSe 2 is a semiconductor, while β-Ag 2Se and β-Ag 2Te are semimetals with an overlap of about 0.1–0.2 eV. The Ag 4d and Au 5d states are strongly hybridized with the chalcogen p states all over the valence bands. β-Ag 2Se and β-Ag 2Te have a very low DOS in the energy range from about −0.1 to +0.5 eV. The calculated effective mass β-Ag 2Se is about 0.1–0.3 m e for electrons and 0.75 m e for holes, respectively.

CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE

The speciation of Au in gold-bearing arsenopyrite (FeAsS) from four gold deposits (Olympiada, Sentachan, Sao Bento and Sheba) was determined by micro-X-ray absorption near-edge structure (XANES) on grains well characterized microscopically and by electron-microprobe and secondary-ion mass spectrometry analyses and images. “Invisible” gold in arsenopyrite occurs in two apparently mutually exclusive chemical forms: chemically bound and elemental. Arsenopyrite from the Sentachan, Sao Bento and Sheba deposits contains chemically bound gold. With comparable constituent electronegativities and a white-line feature in the XANES indicating unoccupied Au 5d-states, but absorption-edge positions comparable to Au 1+ species, the bonding is interpreted as being covalent rather than ionic. The invisible gold in arsenopyrite from the Olympiada deposit, on the o ther hand, occurs as very small particles of Au 0 , probably less than a few nanometers in diameter. Micro-XANES data for the Olympiada and Sentachan arsenopyrite support earlier results obtained by 197 Au Mossbauer spectroscopy on arsenopyrite concentrates. In some arsenopyrite crystals, the gold concentration is closely related to growth zoning. This feature represents c onditions during crystallization and does not correlate with the chemical form of the gold. Similarly, selenium, where present, correlates with gold in some deposits and not in others, irrespective of the gold speciation. The finding of two types of invis ible gold in arsenopyrite from different deposits has beneficial implications for extractive metallurgy.

Characterization of Silica-Supported Pd−Au Clusters by X-ray Absorption Spectroscopy

Silica-supported Pd−Au clusters were characterized by Pd K and Au L3 extended X-ray absorption fine structure (EXAFS) spectroscopy and Au L2,3 X-ray absorption near-edge structure (XANES) spectroscopy. Pd−Au/SiO2 catalysts with Pd/Au atomic ratios of approximately 1:1 were prepared by coadsorption of [Pd(NH3)4][NO3]2 and [Au(en)2]Cl3 (en = ethylenediamine) onto silica gel from pH 7 and pH 10 slurries. EXAFS spectroscopy of the Pd−Au/SiO2 (1.3:1, pH 7) catalyst after in situ reduction at 300 °C indicates the formation of bimetallic clusters exhibiting Pd surface enrichment. The EXAFS results can be approximated using a cluster “decoration” model in which a Au-rich core is covered by a partial monolayer of Pd. In contrast, EXAFS spectroscopy of the Pd−Au/SiO2 (1:1, pH 10) catalyst after in situ reduction at 350 °C indicates the formation of homogeneous Pd−Au alloy clusters. The homonuclear bond distances in these clusters agree closely with the Vegard's Law prediction for a 1:1 Pd−Au alloy; however, the heteronuclear bond distances are ∼0.02 Å shorter. Au L2,3 XANES spectroscopy of the supported Pd−Au clusters evidences a vanishingly small density of unoccupied Au 5d states. Excellent correspondence is observed between the XANES spectra of supported Pd−Au clusters and those of Pd−Au alloy films having similar compositions.

Electronic Band Structures off-Electron Ternary Compounds with an Energy Gap

Recent results of APW band calculations of ThNiSn, ThaNisSb4 and LasAusSb4 are summarized. These compounds are proper reference materials for the study of semiconductor-like property of UNiSn, UsNisSb4, CesAusSb4 and CesPtsSb4. The valence bands consist of the Sn/Sb 5p and the Ni 3d/ Au 5d states. The conduction bands are derived from the Th 6d/La 5d states and are raised by the mixing between the Th 6d/La 5d state and the Ni 3d/ Au 5d state. A band gap appears between the occupied valence bands and the empty conduction bands. On the basis of these results, the ·origin of gap formation in UNiSn, UsNisSb4, CesAusSb4, CesPtsSb4 and isostructural uranium and cerium compounds is discussed.

Theoretical study of the electronic structure of ordered and disordered Cu3Au.

The electronic structure of the ordered phase of ${\mathrm{Cu}}_{3}$Au is investigated using the self-consistent-field (SCF) fully relativistic linear-muffin-tin-orbital method. The electronic structure of the disordered phase is obtained from a fully relativistic Korringa-Kohn-Rostoker coherent-potential-approximation calculation. To show the effects of self-consistency in the disordered phase, we present both non-SCF and SCF results for this phase. These results show that the largest changes in the electronic structure upon ordering are in energy regions away from the Fermi energy. To verify this prediction we have calculated the x-ray-photoemission (XPS) intensities for both phases and compared these to experimental data. It is found that the calculated Al K\ensuremath{\alpha} XPS intensity is in excellent agreement with the data on the ordered system and gives a clear picture of the itinerant nature of the Au 5d states. For the disordered state our results are in agreement with trends seen in angle-resolved photoemission, but are in disagreement with the only angle-integrated XPS data available on the disordered system.

MIXED VALENCE STATE OF Sm ON THE SURFACE OF AMORPHOUS La80-xSmxAu20 ALLOYS

UPS measurements analyzed in the light of XPS spectra yield some information about the inhomogeneous nature of the Sm surface mixed-valence state in amorphous LaSmAu alloys. In addition, the quasi-ionic character expected for these materials is clearly evidenced by the observation of the structure of the Au 5d states. In a recent letter /I/, we reported the re- letter /I/. Let us recall, first, what can be sults of XPS measurements on a series of splat- gained by the combined use of the XPS (A1 Ka =