Observation of atomically displacive transformation out of the boundary-reconstructive phase competition

During the phase transitions, diverse states evolve with multiplex phenomena arising from the critical competition. In this study, a displacive martensitic transformation with a lattice shear distortion was unexpectedly observed at the reconstructive phase boundary that usually connects multiple phases without crystallographic relation, in a Ni-Co-Mn-V all-d-metal Heusler alloy system. Experiments and theoretical calculations suggest that the parent phase becomes increasingly unstable when approaching the phase boundary. The lattice-distorted transformation with moderate first-order nature survives due to the critical phase competition from the structural frustration, in which the comparable energy and the diminished formation preference of different phases emerge. In this critical state, the phase selection including the martensitic transformation can be tuned by external fields such as rapid cooling, annealing, and magnetic field. Our research reveals a novel manner to destabilize the parent phase, through which one could attain new functional materials based on the phase transitions.