Structural and mechanistic revelations on an iron conversion reaction from pair distribution function analysis.

Not simply small particles: pair distribution function analysis yields comprehensive insights into the electrochemical reaction of α-Fe(2)O(3) with lithium. The metallic Fe formed in this reaction was found to be defect-rich nanoparticles that restructure continuously without growing-an unusual characteristic likely linked to its highly reversible capacity.

Efficient simultaneous reverse Monte Carlo modeling of pair-distribution functions and extended x-ray-absorption fine structure spectra of crystalline disordered materials.

An efficient implementation of simultaneous reverse Monte Carlo (RMC) modeling of pair distribution function (PDF) and EXAFS spectra is reported. This implementation is an extension of the technique established by Krayzman et al. [J. Appl. Cryst. 42, 867 (2009)] in the sense that it enables simultaneous real-space fitting of x-ray PDF with accurate treatment of Q-dependence of the scattering cross-sections and EXAFS with multiple photoelectron scattering included. The extension also allows for atom swaps during EXAFS fits thereby enabling modeling the effects of chemical disorder, such as migrating atoms and vacancies. Significant acceleration of EXAFS computation is achieved via discretization of effective path lengths and subsequent reduction of operation counts. The validity and accuracy of the approach is illustrated on small atomic clusters and on 5500-9000 atom models of bcc-Fe and α-Fe(2)O(3). The accuracy gains of combined simultaneous EXAFS and PDF fits are pointed out against PDF-only and EXAFS-only RMC fits. Our modeling approach may be widely used in PDF and EXAFS based investigations of disordered materials.

High-brightness photocathodes through ultrathin surface layers on metals.

We report how ultrathin MgO films on Ag(001) surfaces can be used to control the emittance properties of photocathodes. In addition to substantially reducing the work function of the metal surface, the MgO layers also favorably influence the shape of the surface bands resulting in the generation of high-brightness electron beams. As the number of MgO surface layers varies from 0 to 3, the emitted electron beam becomes gradually brighter, reducing its transverse emittance to 0.06 mm mrad. We suggest the use of such photocathodes for the development of free-electron x-ray lasers and energy-recovery linac x-ray sources.

Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation.

A room temperature solid-state structural transformation was observed in 3 nm ZnS nanoparticles in methanol following the addition of water (Zhang et al., Nature 424, 1025, 2003). Experimental wide angle x-ray scattering (WAXS), x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy measurements show a large increase in crystallinity associated with water addition, in agreement with molecular dynamics (MD) predictions. Here we perform first-shell EXAFS and pair distribution function analysis and whole-nanoparticle calculations of WAXS, EXAFS and XANES to compare structural data with the MD predictions. The predicted WAXS patterns give excellent agreement with data, while the predicted EXAFS and XANES spectra give poor agreement. Relative to WAXS, XANES and EXAFS spectra contain additional structural information related to the distribution of disorder. The discrepancy between the x-ray diffraction and x-ray absorption results indicates that structural disorder is partitioned between interior and surface regions more strongly than predicted in the MD simulations.