Study of Local Structure, Stress and Dynamics in Disordered Materials Using Ab-Initio and Molecular Dynamics Simulation

Understanding the atomic structure and dynamics in structurally disordered systems has been a long-standing and most challenging problem in physics and material science. To begin with, it is difficult to describe disorder quantitatively and to differentiate the degree of disorder from one system to another. The majorities of experimental and theoretical approaches to the study of disordered systems are either transferred directly from the study of crystals or address the problem in the macroscopic scale where the atomic origin of behavior is obscured. First principle atomic level stresses and dynamic pair distribution functions described in this dissertation represent attempts to overcome these limitations of current approaches. They relate system-specific atomic level properties to macroscopic properties such as viscosity and the glass transition temperature. The novel dynamic pair-density-function method effectively explores the dynamics in disordered systems as demonstrated by our discovery of super-localization in high-temperature liquid iron. The dynamic pair distribution function and local stresses are governed by the bonding within the cage of neighboring atoms; they reveal very localized dynamics. We have used atomic level stresses to characterize materials within the local approximation to density functional theory using the Locally Self-consistent Multiple Scattering method. The results of calculations on several crystals, liquids and glasses and radiation damaged bcc iron are presented. Atomic level stress calculations are also used to address the issue of metallic glass formability in the case of Au-Al system. We are advancing the field from qualitative results based on models to system-specific, quantum-mechanics-based calculations of atomic level mechanisms.