Chemical ordering in Pd$_{81}$Ge$_{19}$ metallic glass studied by reverse Monte-Carlo modelling of XRD, ND and EXAFS experimental data

Abstract

Pd$_{81}$Ge$_{19}$ metallic glass was investigated by neutron diffraction, X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) at the Ge K-edge. Large scale structural models were obtained by fitting the three measurements simultaneously in the framework of the reverse Monte Carlo simulation technique. It was found that the experimental data sets can be adequately fitted without Ge-Ge nearest neighbours. Mean Pd-Pd and Pd-Ge distances are 2.80$\pm$0.02 {\AA} and 2.50$\pm$0.02 {\AA}, respectively. The total average coordination number of Pd is 12.1$\pm$0.5 while Ge is surrounded by 10.6$\pm$1.1 Pd atoms. The coordination numbers calculated from partial pair correlation functions were compared to those obtained by Voronoi tessellation method. It was found that the latter technique overestimates the number of nearest neighbours by about 20% due to the significant contribution of distant pairs.