2D Magnetic Janus Semiconductors with Exotic Structural and Quantum-Phase Transitions.

2D magnetic semiconductors are intriguing for their great potential applications in spintronic nanodevices. Despite intensive research for decades, intrinsically 2D magnetic Janus semiconductors are scarce, and their design guidelines remain elusive. Herein we propose new 2D Janus Cr2O2XY (X = Cl, Y = Br/I) ferromagnets with asymmetric out-of-plane structural configurations from ab initio calculations. Abnormally, 2D Janus Cr2O2XY crystals with Pmm2 structures derived from pristine CrOX compounds are dynamically metastable. By introducing novel structural phase transitions, we generated new Pma2 phases with lower total energy and great dynamical stability. These new Janus Cr2O2XY monolayers are intrinsically ferromagnetic semiconductors and could be easily synthesized from experiment. Most interestingly, exotic quantum-phase transitions from the ferromagnetic semiconductor to the antiferromagnetic metal/semiconductor could be achieved in the Cr2O2ClI monolayer by applying compressive strains. Our study provides an alternative strategy to design new Janus Cr2O2XY monolayers and will inspire further investigations on relevant materials for electronic and spintronic applications.