9 - Structure of Melt and Liquid Alloys

Abstract

In the study of nucleation and growth of crystals from the melt, it is mandatory to determine accurately the structure of the liquid phase up to the atomic level. However, the analysis of the local atomic structure of liquids is challenging because the liquids are locally ordered but disordered above a few shells of neighbors. As a consequence, the diffraction pattern of liquids contains a limited amount of information on a structure that requires, in principle, many parameters to be described, in contrast with crystalline materials. Nonetheless, many techniques have been developed, among which the scattering of neutrons and X-rays and the X-ray absorption technique (EXAFS) are the most widely used. Their output is a spherically averaged scattered intensity I(θ) normalized to the structure factor S(q) that gives, by Fourier transformation, the pair correlation function g(r), i.e., the distribution of interatomic distances. The diffraction analysis has to be complemented by many characterization techniques: measurements of various physical parameters, (density, conductivity, thermal properties), and neutron and X-ray inelastic scattering spectra. In addition, the computer simulation of the structure of liquids contributed considerably to our understanding of the liquid phases (e.g., Monte Carlo simulation, ab initio molecular dynamics, reverse Monte Carlo analysis). We review the different approaches of the study of liquids for various systems and scales from the simplest metallic structures to the more complex semiconducting alloy structures, under different sample environments. We emphasize the importance of large-scale facilities: neutron sources and X-ray synchrotrons. Some specific examples are developed.