Micro Reverse Monte Carlo Approach to EXAFS Analysis

Abstract

We have developed a new Reverse Monte Carlo code, called the micro Reverse Monte Carlo (m-RMC), which is applicable to structure analysis of nanomaterials and surface species. In the m-RMC, Reverse Monte Carlo is applied to an ensemble of replica files, each of which contains one molecule or one small cluster, because the Extended X-ray Absorption Fine Structure (EXAFS) is sensitive to short-range structures and has negligible interaction between molecules or clusters. We apply the m-RMC to face-centered cubic metals (Cu, Pd, and Pt) and discuss the advantages, validation, and problems of the m-RMC. The bond distance and some cumulant coefficients can be determined from the EXAFS using m-RMC. Some 50-100 replica files are sufficient to reproduce the EXAFS oscillations and radial distributions. The bond distance can be determined, including the asymmetric distributions, by m-RMC. We also apply m-RMC to α-MoO3 and Au clusters. The m-RMC analysis of MoO3 shows that three radial distribution peaks appear corresponding to three types of MoO bonds. The m-RMC analysis of Au cluster indicates the presence of Au55 cuboctahedral structure with Au-Au distance at 0.288 nm. We obtain a 3 D image of Au55 nanocluster from the unified file. The m-RMC method can be applied to the analysis of the EXAFS for chemical systems with appropriate care.

Thorough Search Analysis of Extended X-ray Absorption Fine Structure Data for Complex Molecules and Nanomaterials Applications

“…Both oscillations agreed well with an R-factor of 0.0018. These parameters determined by the TS analysis corresponded well with those of the CF results within errors and those obtained from m-RMC results, as shown in Table 2 [8]. Figure 4(b−e) presents histograms of the structural parameters 0 2 , ΔE 0 , r, and σ 2 .…”

“…To overcome the problems associated with the CF analysis, Fujikawa et al [8] developed the micro reverse Monte Carlo (m-RMC) method and successfully determined the structure of α-MoO3 without any assumptions of initial structural parameters. The m-RMC is a kind of reverse Monte Carlo approach [9−14].…”

“…The reverse Monte Carlo approach tries to find the structure with the random walk of atoms in a large ensemble to seek the low R-factor using the Metropolis method. The m-RMC method uses a large number of ensembles of clusters, for which the EXAFS oscillations are calculated and determines the local structure [8].…”

“…The second is α-MoO 3 as a more complex case. The advantages and disadvantages of the TS analysis are discussed through comparison with the CF analysis [7] and the m-RMC method [8].…”

Structure and atomic correlations in molecular systems probed by XAS reverse Monte Carlo refinement

“…9,10 Application of the RMC method for XAS data-analysis was already suggested by Gurman and McGreevy 11 and several other studies have been published so far (see, for example, Refs. [12][13][14][15][16][17][18][19][20]. In particular, the RMC-GNXAS method is designed to apply the RMC algorithm simultaneously to diffraction and XAS data, allowing for the construction of a tridimensional model of the atomic system compatible with the observables under consideration.…”