Strain-induced orientational dependent ferroelectric phase transition in anisotropic NbOCl2 flakes

Strain engineering demonstrates remarkable precision in inducing phase transitions, as well as high orientability, enabling tunable phase transitions with low energy consumption and rapid response. NbOCl2, an emerging ferroelectric (FE) layered two-dimensional (2D) material, exhibits pronounced in-plane FE properties and demonstrates a significant anisotropic second harmonic generation response. Here, we demonstrate that the FE phase transition in NbOCl2 can be modulated by applying strain relative to its intrinsic lattice orientation. It has been discovered that the strain-induced FE phase transition in NbOCl2 crystals depends on the direction of the polar axis. Specifically, when strain is applied along the polar axis and reaches a minimal threshold of just 0.65%, it induces a transition from the FE phase to the antiferroelectric phase. By releasing the strain, NbOCl2 reverts to the FE phase, enabling a tunable phase transition along the polar axis. Furthermore, it was discovered that the challenge in inducing phase transition behavior by applying uniaxial strain along the nonpolar axis is due to the difficulty in effectively coupling the strain field to the key atomic interaction system that determines the FE properties. This work not only provides valuable strategies and insights for inducing reversible phase transitions in other 2D materials but also establishes a robust foundation for the development of FE memory devices with enhanced directional controllability.