Pressure-mediated structural phase transitions and ultrawide indirect-direct bandgaps in novel rare-earth oxyhalides

Ultrawide bandgap semiconductors are fundamentally important in solid-state lighting, transparent electrodes and power electronics, but their 2D forms are rarely reported and less studied. By means of ab initio simulations, we predict new trigonal YOBr and monolayered crystals with ultrawide bandgaps and exceptional properties. It is demonstrated that trigonal YOBr is energetically, dynamically and mechanically stable and shows lower energy compared with other known experimental phases. We present that, under hydrostatic pressure, the bulk YOBr crystal undergoes a structural transformation from R3m to P4/nmm, accompanied by an indirect-direct band transition. By further exploring relevant metal oxyhalides MOX (M = Sc/Y and X = Cl/Br), we suggest that, owing to the small exfoliation energy, the monolayers promise experimental fabrication by mechanical cleavage, as for graphene. These MOX monolayers possess excellent stability, large bandgaps and high carrier mobilities. We reveal interesting indirect-direct bandgap transitions in uniaxially strained ScOCl and trigonal YOBr monolayers. In addition, we highlight that remarkable ultraviolet light absorption and appreciable band edges render these MOX monolayers great candidates for potential applications in UV-electronics and photocatalysis. Our findings open a new avenue to explore phase transitions in rare-earth oxyhalides under pressure/strain and provide promising ultrawide-bandgap semiconductors for future optoelectronic devices.