Alexei L. Ankudinov

Sensitivity of Pt x-ray absorption near edge structure to the morphology of small Pt clusters

A theoretical study of the sensitivity of Pt L3 x-ray absorption near edge structure (XANES) to the size and shape in small Ptn clusters is reported. Calculations, based on a full multiple scattering, self-consistent field, real-space Green’s function approach implemented in the ab initio FEFF8 code, show that XANES provides a characteristic signature of cluster shape. For example, the calculated white line intensity exhibits a large variation for small cluster sizes and geometry, but becomes independent of cluster size for large clusters. A strong polarization dependence of the white line is predicted for two-dimensional clusters. For three-dimensional clusters the polarization dependence is smaller, but can be used as a measure of the “flatness” of a cluster. A series of semirelativistic all-electron, full potential density functional calculations was also performed for several Ptn clusters. These calculations show the existence of intrinsic static disorder in these clusters due to nonisotropic shrinkag...

Crystallization-induced short-range order changes in amorphous GeTe

By means of x-ray absorption fine structure and Raman scattering spectroscopies we demonstrate that the structure of amorphous GeTe is likely to be a mixture of 4(Ge):2(Te) and 3(Ge):3(Te)-coordinated structural units. Upon crystallization, a rhombohedral (distorted rocksalt) structure is established with about 10% of vacancies occurring on Ge sites. The vacancies are believed to play an important role in determining the ratio of 3(Ge):3(Te) and 4(Ge):2(Te) structural units.

Progress and challenges in the theory and interpretation of X-ray spectra

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure (XAFS). Rapid progress is now being made in understanding X-ray absorption near-edge structure (XANES). This presentation reviews the developments in this field by many groups leading up to the current state of the art. These developments have led to several ab initio codes, such as FEFF, which yield results comparable to experimental results for XAFS, and permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. The review begins with a summary of the key theoretical developments that are essential for achieving a quantitative agreement with experiment for XAFS. The same high-order multiple-scattering (MS) theory of XAFS can also give an approximate treatment of XANES, but this approach can fail close to an edge, where full MS calculations are often necessary. However, a fully quantitative treatment of XANES remains challenging, largely as a result of a number of many-body effects, e.g. the approximate treatment of the core-hole, multiplet effects, the photoelectron self energy and inelastic losses. Finally, natural extensions of the theory to other spectroscopies, such as anomalous X-ray scattering, DAFS (diffraction anomalous fine structure) and XMCD (X-ray magnetic circular dichroism) are briefly discussed. These developments are illustrated with a number of applications.

Inelastic scattering from core electrons: A multiple scattering approach

The real-space multiple-scattering approach is applied to model nonresonant inelastic scattering from deep core electron levels over a broad energy spectrum. This approach is applicable to aperiodic or periodic systems alike and incorporates ab initio, self-consistent electronic structure and final state effects. The approach generalizes to finite momentum transfer a method used extensively to model x-ray absorption spectra (XAS), and includes both near-edge spectra and extended fine structure. The calculations can be used to analyze experimental results of inelastic scattering from core electrons using either x-ray photons or electrons. In the low momentum transfer region (the dipole limit), these inelastic loss spectra are proportional to those from XAS. Thus, their analysis can provide similar information about the electronic and structural properties of a system. Results for finite momentum transfer yield additional information concerning monopole, quadrupole, and higher couplings. Our results are compared both with experiment and with other theoretical calculations.

Why phase-change media are fast and stable : A new approach to an old problem

Present-day multimedia strongly relies on re-writable phase-change optical memories. We find that, different from current consensus Ge2Sb2Te5 (GST), the material of choice in digital versatile discs–random access memory (DVD-RAM), possesses a structure similar to ferroelectric GeTe, namely that Ge and Sb atoms are located off-center giving rise to a net dipole moment. Amorphisation of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in the short-range order. We demonstrate that the order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions cycles.

New developments in NEXAFS/EXAFS theory

Abstract Recent advances in the theory of X-ray absorption fine structure (XAFS) are reviewed. Modern ab initio multiple-scattering (MS) calculations now provide an accurate, unified treatment of XAFS, encompassing both EXAFS (extended-XAFS), NEXAFS (near-edge XAFS) and XANES (X-ray absorption near-edge structure). Combined with multi-path analysis techniques, such calculations now permit accurate structural determinations well beyond the first near-neighbor. These calculations also describe near-edge features such as σ* shape resonances and white lines. Implications for catalyst research are briefly discussed.

Single configuration Dirac-Fock atom code

A single configuration version of the multiconfigurational Dirac-Fock atomic code of Desclaux has been developed into an automated FORTRAN 77 subroutine, requiring only the atomic number as input. The output contains energies, wavefunctions, densities and other data similar to that in atomic structure tables.

Theoretical Interpretation of XAFS and XANES in Pt Clusters

This paper first briefly summarizes the dramatic progress over the past decade both in fundamental theory and in the interpretation of XAFS and XANES. These developments have led to several ab initio codes such as FEFF for simulating XAFS and XANES, together with compatible analysis codes which permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. As an example of relevance to catalysis, we discuss recent work which interprets the Pt L-edge XANES of PtX clusters based on the self-consistent FEFF8 code. For pure Pt clusters, we find that self-consistency is important in determining the variation of XANES with cluster size. For PtCl clusters, we show that the presence of a Cl–Pt bond leads to a “hybridization peak,” i.e., a peak in the Cl d-density of states (dDOS) mixed with Pt d-states, which can be used as a measure of Cl content. For Pt–H clusters, we show that hydrogen addition is well correlated with the growth of a broad shoulder above the white line. We find that this feature can be attributed largely to AXAFS, i.e., to a change in the atomic background absorption. We also analyze the effect of a support, in terms of model calculations for a realistic Pt6 cluster within a zeolite-LTL pore.

Hole counts from X-ray absorption spectra.

The interpretation of X-ray absorption spectra in terms of electronic structure has long been of interest. Hole counts derived from such spectra are often interpreted in terms of free-atom occupation numbers or Mülliken counts. It is shown here, however, that renormalized-atom (RA) and cellular counts are better choices to characterize the configuration of occupied electron states in molecules and condensed matter. A projection-operator approach is introduced to subtract delocalized states and to determine quantitatively such hole counts from X-ray absorption spectra. The described approach is based on multiple-scattering theory (MST) and on atomic calculations of a smooth transformation relating the X-ray absorption spectra to local projected densities of states (LDOS). Theoretical tests for the s and d electrons in transition metals show that the approach works well.

Atomic-XAFS and XANES

X-ray-absorption near edge structure (XANES) poses many theoretical challenges and a general quantitative theory remains elusive. We have studied structural contributions to XANES based on high-order multiple-scattering (MS) calculations, including X-ray polarization. Ab initio MS calculations of XANES are presented using a code FEFF6X, that includes estimates of the amplitude reduction factor S 0 2 . White-lines and continuum shape resonances in XANES are described quantitatively. We have observed that the atomic background absorption contains oscillatory structure which can play an important role in XANES. Calculations and measurements confirm the existence of AXAFS and show that it can dominate contributions such as multi-electron excitations