Local structure of solid and liquid gold probed by reverse Monte Carlo analysis of X-ray absorption data.

Abstract

The increase in computational capabilities of modern computers has allowed us to develop new accurate methods for EXAFS (extended X-ray absorption fine structure) data-analysis. In particular, the RMC-GNXAS package provides models of the three-dimensional structure applying the reverse Monte Carlo (RMC) method to EXAFS of condensed or molecular systems. Simulations of the EXAFS signals within RMC-GNXAS are based on accurate multiple-scattering (MS) simulations accounting for relativistic effects. The method is implemented to include MS signals related to pair and triplet distributions, using different data sets and long-range constraints provided by complementary techniques (diffraction). In this work we have validated this approach studying the local structure of solid and liquid Au, providing also a benchmark for analysis of functional materials based on heavy noble atoms such as Au. An accurate reconstruction of the pair and triplet distribution functions in solid and liquid Au was obtained and compared with literature data. We show that the inclusion of the contribution of MS signals related to three-body configurations is especially important in solid Au at low temperatures. In liquid Au, we obtained the pair distribution function and analyzed the bond-angle distribution. We applied common-neighbor analysis to identify the local symmetry of the atomic configurations. Liquid Au is found to follow the trend of other close-packing liquids for which the dominant structures are distorted or defective icosahedral configurations.