<Abstrate>

Crystallographic imperfections significantly alter material properties and their

response to external stimuli, including solute-induced phase transformations.

Despite recent progress in imaging defects using electron and X-ray techniques, 

in situ three-dimensional imaging of defect dynamics remains challenging. Here, we

use Bragg coherent diffractive imaging to image defects during the hydriding phase

transformation of palladium nanocrystals. During constant-pressure experiments

we observe that the phase transformation begins after dislocation nucleation close

to the phase boundary in particles larger than 300 nm. The three-dimensional

phase morphology suggests that the hydrogen-rich phase is more similar to a

spherical cap on the hydrogen-poor phase than to the core–shell model commonly

assumed. We substantiate this using three-dimensional phase field modelling,

demonstrating how phase morphology affects the critical size for dislocation

nucleation. Our results reveal how particle size and phase morphology affects

transformations in the PdH system.
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