Study of local icosahedral ordering in liquid and undercooled liquid copper

Useful information about the average local geometry in simple liquids can be obtained by the X-ray absorption spectroscopy (XAS). Advanced XAS data-analysis methods based on multiple-scattering simulations are able to give accurate information about the local pair and triplet distribution. In this work we give a full account of a recent experiment aimed to study the local geometry in liquid and undercooled copper, a typical close-packed liquid metal. The local structure of these liquids is known to be significantly different from that of corresponding crystalline systems possibly containing icosahedral local ordering forbidden in periodic structures. Signatures of shortrange fivefold ordering have been previously found in liquids but current knowledge is limited to the pair distribution function g(r). State-of-the-art experimental and data-analysis techniques have been used to study accurate XAS Cu K-edge data. The application of a Reverse Monte Carlo refinement to combined XAS and diffraction data provided us with an ensemble of realistic tridimensional models of the liquid structure. A detailed geometrical analysis of these models has shown that experimental data are compatible with a broad distribution of local geometries and that the fraction of nearly-icosahedral configurations is around 10% in liquid and undercooled liquid Cu, in agreement with recent molecular-dynamics calculations stimulated by present results. (c) 2007 Elsevier B.V. All rights reserved.