Theoretical advancements towards understanding crystalline metastability during materials synthesis

Abstract

Predicting the conditions in which a compound adopts a metastable structure when it crystallizes out of solution is an unsolved and fundamental problem in materials synthesis, and one which if understood and harnessed, could enable the rational design of synthesis pathways towards or away from metastable structures. Although metastable phases are ubiquitous in both nature and technology, only a heuristic understanding of their underlying thermodynamics and formation mechanisms exists. In this thesis, we aim to address two important outstanding questions regarding the fundamental nature of metastability: Which metastable phases can form? Under which conditions will they form? We will employ a variety of computational and theoretical approaches to elucidate quantitative insights to these two questions. To better predict which metastable materials can be made, we first seek to understand the metastable materials that have been made. We data-mine the Materials Project, a high-throughput database of DFT-calculated energetics of ICSD structures, to explicitly quantify the energy scale of metastability for 29,902 inorganic crystalline phases. We reveal the influence of chemistry and composition on the accessible range of crystalline metastability, and identify motifs characteristic of highly metastable compounds. We further assert that not all low-energy metastable materials can necessarily be made, and argue for a concept of "remnant metastability" - that observable metastable phases are remnants of thermodynamic conditions where they were once the lowest free-energy phase. Recently, exciting thermochemistry experiments have demonstrated that for many compounds, as metastability of a phase increases, its surface energy decreases. This effect is significant enough to trigger a reversal of relative phase stability in nanoparticles. Because nucleation and growth starts at the nanoscale, we hypothesize that the direct precipitation of metastable phases during crystallization may be 'remnant metastability' of size-dependent nanoscale phase stability. We develop algorithms for the automated, efficient, and high-throughput calculation of surface energies via DFT. We combine these algorithms with prior theoretical frameworks to predict solid-aqueous equilibria, enabling the calculation of nucleation barriers of competing polymorphs as a function of solution chemistry, thereby predicting the solution conditions governing polymorph selection. We apply this approach to resolve the long-standing 'Calcite-Aragonite Problem' - the observation that calcium carbonate precipitates as the metastable aragonite polymorph in marine environments, rather than the stable phase calcite - which is of tremendous relevance to biomineralization, carbon sequestration, paleogeochemistry, and the vulnerability of marine life to ocean acidification. We identify a direct relationship between the calcite surface energy and solution Mg/Ca ion concentrations, showing that the calcite nucleation barrier surpasses that of metastable aragonite in solutions with Mg/Ca ratios consistent with modern seawater, allowing aragonite to dominate the kinetics of nucleation. Our ability to quantify how solution parameters distinguish between polymorphs marks an important step towards the ab initio prediction of materials synthesis pathways in solution.