Kavli Affiliate: David T. Limmer
| Summary:
Pathways and structural dynamics of phase transformations impact performance of materials in energy and information storage technologies. Palladium hydride ($mathrmPdH_x$) nanocrystals are an ideal model system for studying solute-induced phase transformations, where elastic energy from lattice mismatch between $α$-$mathrmPdH_x$ and $β$-$mathrmPdH_x$ phases is often considered a key to determining the transformation pathways. $α/β$-$mathrmPdH_x$ interfacial elastic energy is affected by the confined geometry of a nanocrystal. However, how nanocrystal geometry influences phase transformation pathways is largely unknown. Using in situ liquid phase transmission electron microscopy, we directly visualize hydrogenation in Pd nanocrystals with two geometries — a nanocube and a hexagonal nanoplate. Both follow similar sequences of an initially curved nucleus, interface flattening, and reverse-stage nucleation; however, their evolving $α/β$-$mathrmPdH_x$ interfaces exhibit geometry-dependent crystallographic alignments. In nanocubes, $100$-aligned configurations conform to static elastic energy ordering, representing a pathway that maintains a local mechanical equilibrium, whereas nanoplates display both $110$- and $211$-aligned interfaces. Theoretical simulations show that geometry determines the accessibility of alternative phase transformation pathways as the system is driven far from equilibrium during hydrogenation. These findings identify geometry as a fundamental parameter for directing phase transformation pathways, offering design principles for accessing atypical configurations and improving properties of intercalation-based devices.
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