Understanding structure-property relationships in beta-eucryptite through atomistic simulations

Abstract

Beta-eucryptite (LiAlSiO4) has drawn widespread commercial and academic interests due to its exotic physical properties, mainly, negative thermal expansion. The present study employs a combination of density functional theory (DFT) calculations and atomistic simulations to identify the atomic-scale mechanisms underlying three intriguing phenomena in beta-eucryptite: (a) negative bulk thermal expansion (NTE), (b) radiation tolerance, and (c) structural transformations under moderate applied pressure. Using DFT calculations, we resolved a long-standing discrepancy in the literature concerning the sign of linear compressibility of beta-eucryptite parallel to the c-axis, [chi]c. Our DFT calculations that are in excellent accordance with recent ultrasonic experiments have led to the conclusion that [chi]c has a positive value, as opposed to a negative value reported by earlier direct measurements. Furthermore, this study suggests that the NTE behavior of beta-eucryptite occurs due to tetrahedral tilting and cation disordering rather than elastic effects. To describe the atomic interactions in our atomistic simulations, we parameterized a reactive force field (ReaxFF) for Li- Al-Si-O system using DFT calculated structural properties of several bulk phases of silicates, aluminates, and oxides, and various representative clusters. These parameters were found to have good transferability, as they provide an accurate description of (a) structural, thermodynamic and mechanical properties of numerous condensed phases, (b) phase transformations, as well as (c) single species systems (e.g., Li-, Al- metals, Si) with applications to oxidation and reduction reactions. Molecular dynamics simulations based on ReaxFF showed that upon exposure to neutron radiation, beta-eucryptite largely retains its long-range order while exhibiting tetrahedral rotation, change in O-coordination around Al/Si atoms, and Li disordering. Upon annealing radiated beta-eucryptite, most of the under-coordinated Si-polyhedra formed during radiation regained their tetrahedral coordination via a mechanism involving tilting of Al- and Si- centered polyhedra. Finally, our ReaxFF based metadynamics simulations revealed that pressure-induced amorphization (PIA) in beta-eucryptite is governed by atomic-scale processes similar to those observed in response to temperature changes and radiation exposure. The results presented in this thesis show a general link between NTE, radiation tolerance, and PIA in flexible framework structures.