Direct observation on single-crystal-to-single-crystal transformation via lattice-matched nucleation and growth

Single-crystal-to-single-crystal (SCSC) phase transformation has been showing its great significances for controllable synthesis of advanced materials. From this point of view, the atomic-scale understanding of structural dynamics during SCSC transformation is of fundamental criticality for its utilizations in many disciplines, which, however, remains a grand challenge due to the lack of direct and effective experimental probes. Here, we report a complete observation regarding atomic-scale mechanisms for nucleation and growth of a hexagonal phase in a monoclinic matrix during a heating-induced structural transformation using in situ transmission electron microscopy. The overall process consists of several distinct steps; namely, defect generation, phase nucleation, phase growth and propagation. In particular, the new hexagonal phase is found to nucleate at defect-rich high-energy surface regions of the matrix, and subsequently grow in a lattice-matched manner between the old monoclinic and new hexagonal phases, thus maintaining the single-crystalline nature of the product phase during the whole transformation, even in the case of discrete nucleation at multiple sites. Such findings provide crucial insights for the understanding of microscopic mechanisms and kinetics of solid-state phase transitions.