Interpretation of X-ray absorption spectra of As(III) in solution using Monte Carlo simulations.

We performed X-ray absorption spectroscopy measurements on the arsenic K-edge of As(III) in solution under acidic conditions. Extended X-ray absorption fine structure (EXAFS) and X-ray near edge structure (XANES) spectra were compared with theoretical calculations which use local atomic structure configurations, either derived from density functional theory (DFT) energy minimization (EM) calculations or based on classical Monte Carlo (MC) simulations, for a As(OH)3 cluster surrounded by water molecules. The nearest arsenic-oxygen distances obtained from the fit of the XAFS spectra are consistent with the distances present in configurations derived from Monte Carlo simulations but not with those obtained from DFT-EM calculations. Calculations of XANES using either DFT-EM or the average configuration obtained from MC simulations do not reproduce the XANES spectra in the vicinity of the absorption edge. However, specific local atomic structural configurations of the As(OH)3 and water molecules, obtained from MC simulations, which show some ordering of water molecules up to 5 Å from the arsenic, reproduce qualitatively the experimental spectra. These results highlight the capability of XANES to yield information about hydration of ions in solution.

Tris(tetra-butyl-ammonium) tris-(nitrato-κO,O')tetra-kis-(thio-cyanato-κN)thorium(IV).

The title compound, (C(16)H(36)N)(3)[Th(NCS)(4)(NO(3))(3)], was obtained from the reaction of Th(NO(3))(4)·5H(2)O with (Bu(4)N)(NCS). The Th(IV) atom is in a ten-coordinate environment of irregular geometry, being bound to the N atoms of the four thio-cyanate ions and to three bidentate nitrate ions. The average Th-N and Th-O bond lengths are 2.481 (10) and 2.57 (3) Å, respectively.

Influence of the local atomic structure in the X-ray absorption near edge spectroscopy of neptunium oxo ions.

Experimental L(III) X-ray absorption near edge structure (XANES) spectra of the distorted octahedral neptunium oxo ions NpO(2)(OH)(4)(2-), NpO(4)(OH)(2)(3-), and NpO(6)(6-) are interpreted using relativistic full multiple scattering calculations of the X-ray absorption process. In this series of compounds, the neptunium cation exhibits two different oxidation states, VI and VII, with coordination spheres from di- to tetra oxo for the first two compounds. The comparison between calculated XANES spectra using the feff code and experimental ones shows that the main features in the spectra are determined by the local coordination around the actinide metal center. Furthermore, the projected density of electronic states (DOS) calculated from the XANES simulations using the feff code are compared to calculations using ADF code. They are both discussed in terms of molecular orbitals and qualitative evolution of bonding within this series of compounds.

Local atomic structure of partially ordered NiMn in NiMn/NiFe exchange-coupled layers: 2. Electronic structure calculations.

Local electronic and magnetic structure calculations for NiMn exchange bias alloys are reported for clusters containing NiMn in both the chemically disordered face-centered cubic and the chemically and magnetically ordered L1(0) phases. The results of these calculations are consistent with our local structure measurements that point toward the existence of nanometer-scale ordered clusters at the beginning stages of chemical ordering. The spatial dependence of both the local density of states and the magnetization is strongly influenced by the existence of magnetic order on short length scales, giving rise to an inhomogeneous profile for these quantities across the material with the greatest change at the interface that is still small enough within the domain to imply that the magnetization is still highly developed.