C.D. Garner

THE STRUCTURE OF AMORPHOUS COPPER SULFIDE PRECIPITATES : AN X-RAY ABSORPTION STUDY

Abstract X-ray absorption (XAS) and X-ray photoelectron spectroscopy (XPS) analysis of copper sulfide precipitated from aqueous solution at ambient temperatures reveals the existence of a metastable primitive structure that ages to a structure with the characteristics of amorphous covellite; XAS provides direct structural information on the structure of these amorphous copper sulfides. Extended X-ray absorption fine structure (EXAFS) analysis of the primitive structure demonstrates the presence of disulfide (S2-groups) and a Cu S interaction of 2.8A: the latter is not found in covellite. Copper K-edge X-ray absorption near-edge structure (XANES) indicates the dominance of 3- over 4-coordinate Cu in the primitive phase, while Cu L3-edge spectra reveal only Cu(I) to be present in all precipitates formed. XPS data confirm the presence of only Cu(I) and reveal that three types of S are present. Upon ageing, the primitive structure transforms to one with the characteristics of covellite, and this transformation involves the reordering of the S2- and Cu3S CuS3-layers. Development of the primitive phase from either a wurtzite-like structure or planar Cu3S CuS3-layers is possible, with the structural evolution driven by the antipathy of Cu(II) for tetrahedral coordination and anomalous electron densities in the metastable structures.

X-RAY-ABSORPTION NEAR-EDGE SPECTRA OF TRANSITION-METAL DISULFIDES FES2 (PYRITE AND MARCASITE), COS2, NIS(2) AND CUS2, AND THEIR ISOMORPHS FEASS AND COASS

Metal K- and L3-, sulfur K- and arsenic K- and L3-edge X-ray absorption near-edge spectra of a series of metal disulfides, FeS2 (both pyrite and marcasite), CoS2, NiS2, and CuS2, and their isomorphs, FeAsS and CoAsS, are presented. The features in this region of these spectra are interpreted using band structure and molecular orbital calculations in terms of the transitions from the 1s or 2p3/2 state to unoccupied states. The 3d transition metal L3-edge spectra of these materials show dependence on the degree of multiplet splitting in the final state, and thus offer less information on the electronic ground state. There are substantial differences in the spectra of the isostructural materials, whereas the spectra of the isotopes pyrite and marcasite show several similarities, illustrating the dependence of near-edge region on electronic structure.

X-ray absorption studies of metal complexes in aqueous solution at elevated temperatures

Abstract Experimental methods for performing X-ray absorption spectroscopy (XAS) on metal complexes in dilute aqueous solutions at temperatures up to 573 K are presented. The solutions are contained in silica tubes for study by X-rays of energies greater than 16 keV. For experiments using X-rays of energies in the range 6.5–16 keV, we have developed a titanium cell with Kapton windows. These cells can be heated in an aluminium oven up to 573 K. Using these methods we have studied the tetrahedral: octahedral equilibrium for cobalt(II) in aqueous chloride solutions, Na 2 MoO 4 · 2H 2 O in aqueous solution and cadmium(II) in aqueous chloride solutions. For cobalt(II) in chloride concentrations between 5 and 7 m we have found evidence that [CoCl 4 ] 2− is not the only important tetrahedral species. In molybdate(VI) solutions, [MoO 4 ] 2− is the predominant species over the 1 temperature range 298–573 K. Cadmium(II) in aqueous chloride solutions shows two major cadmium complexes [Cd(OH 2 ) 6 ] 2+ and [CdCl 4 ] 2− . Bond distances for the principal species in these solutions are presented and are found to be in general agreement with those obtained from X-ray crystallographic studies on related solids.

Coordination sites of metals in tetrahedrite minerals determined by EXAFS

Abstract Synchrotron radiation has been used to collect Cu, Ag, Fe, Cd, and Sb K-edge EXAFS data from a series of naturally occurring and synthetic tetrahedrite, (Cu, Ag)10(Cu, Fe, Zn, Cd)2Sb4S13, minerals. Analysis of the spectra has provided direct evidence about the coordination environment of the various metals, and revealed which sites in the structure each will preferentially occupy. The results complement crystallographic data by giving information about each element in sites occupied by more than one cation, whereas crystallography can only reveal the average geometry of each site. Data from the copper and silver EXAFS have demonstrated that the silver goes into trigonal rather than tetrahedral coordination sites, and in the case of one silver-rich sample a close silver-antimony contact has been identified which provides evidence about the nature of a compression of the structure which is found in this sample. In synthetic samples the iron mainly occupies tetrahedral sites, but in natural tetrahedrites of low silver content, can also go into the trigonal site. The cadmium is tetrahedrally coordinated, and the antimony has threefold tetrahedral geometry with a vacant site, although in the silver-rich sample the antimony-silver interaction is also observed. The results have shown the value of multi-element EXAFS spectroscopy in the investigation of a chemically and structurally complex mineralogical system.

A XAS study of the structural changes undergone by amorphous copper sulfides when precipitated from solution

Abstract Amorphous copper sulfide precipitates, synthesised from Cu(NO 3 ) 2 solution and H 2 S/Na 2 S, have been studied by XAS. They all contain Cu(I) and age to covellite, but their initial form depends on the concentration of the Cu(II) solution. Low concentration solutions give rise to a brown sol, which contains trigonally coordinated Cu(I), while more concentrated solutions produce a black floc, which has a close CuCu interaction of 2.66 A similar to that in anilite, Cu 7 S 4 and low chalcocite, Cu 2 S.

EXAFS studies of metallothionein

Extended x-ray absorption fine structure (EXAFS) has been used to investigate the metal environment in a number of metallothioneins (MT). In all cases, the primary coordination sphere consists of sulphur atoms. In some cases, evidence for cluster formation is obtained.

Nickel L-edge X-ray absorption study of nickel phosphine halide complexes

Abstract Nickel L III,II absorption spectra of [NiX 2 P 2 ] or [NiX 2 (PP)] (where X = Cl, Br and P = PEt 3 , PPh 3 and PP = dppe) are shown to be very sensitive to the local Ni environment and able to differentiate between tetrahedral, cis -square planar and trans -square planar geometries.