Defect-mediated structural phase transition in Ta2NiSe5 visualized via in situ TEM

Abstract

Extensive experimental and theoretical efforts have been devoted to elucidating the origin of either structural or electronic instability in the formation of excitonic state in Ta2NiSe5, whereas the effects of lattice defects on actual structural phase transition in Ta2NiSe5 remain elusive. Here, we perform in situ transmission electron microscopy (TEM) experiments to investigate the structural phase transition of Ta2NiSe5 thin films, with a particular focus on the influence of microstructural features and lattice defects on the phase transition. In the cyclic in situ heating-cooling TEM experiments, we track the reversible orthorhombic to monoclinic (vice versa) phase transitions via twinning variant contrast in TEM images and via changes in reflection splitting in diffraction patterns. We further investigate the effects of dislocations and grain boundaries on the phase transition of Ta2NiSe5 thin films. Our findings reveal that individual dislocations can inhibit the transition from monoclinic to orthorhombic phases while simultaneously promoting the reverse transition from orthorhombic to monoclinic phases. Furthermore, we observe a phenomenon of metastable to stable phase transition within grain-boundary-confined regions of Ta2NiSe5 thin films. These in situ TEM observations offer a comprehensive microscopic perspective on defect-mediated phase transitions in Ta2NiSe5. This insight may extend to other varieties of transition-metal chalcogenides, thus implying a broader applicability.