Chemistry Seminar – Dr. Micah Prange
About the event
Speaker: Dr. Micah P. Prange, Geochemistry Group, Pacific Northwest National Laboratory
Host: Dr. Xiaofeng Guo
Title: Decoding structural details of geologically relevant solid phases using ab initio simulations
Abstract: Understanding and controlling the incorporation of trace metal impurities in earth abundant solid host phases is a marquee challenge for geochemistry, with applications concerned with sequestering undesirable species, separating desired ones, or tuning the properties of the host. In all cases, the thermodynamics and energetics of the incorporation reactions are determined by the structural interplay of the impurity element and the host phase. Elastic and inelastic photon scattering experiments like X-ray pair distribution function (X-ray PDF) analysis and extended X-ray absorption fine structure (EXAFS) provide complementary probes of the host and dopant that in principle give a detailed picture of the structural changes accompanying doping. However, structural determinations in such cases are complicated by the inevitable configurational disorder that accompanies incorporation. This problem becomes larger for aliovalent substituents in crystals, which bring additional disorder associated with charge compensating defects, and, especially, amorphous hosts. Faithful simulations of equilibrium thermal motion using ab initio molecular dynamics can improve the situation by allowing the unravelling of configurational and thermal disorder contributions to experimental observables in these challenging situations. In this talk, I will describe methods and viewpoints developed by the geochemistry group at Pacific Northwest National Laboratory over the last decade that use ab initio molecular dynamics simulations as virtual samples for the simulation of EXAFS and PDF measurements as well as other experimental data, like nuclear magnetic resonance (NMR) spectroscopy and neutron scattering (PDF and INS). I will highlight work on ((oxy)hydr)oxide and carbonate solids that demonstrates that these methods can be used to deduce structural details beyond what can be ascertained using traditional analyses that either do not use theoretical structures (like EXAFS shell fitting) or use static structures for interpretation.