Pressure-modulated anisotropic optical responses in van der Waals NbOCl2

NbOCl2 is an emerging layered van der Waals material with prominent anisotropy. In this work, we systematically investigate the anisotropic optical response and structural evolution of NbOCl2 under pressure up to 50 GPa by using polarized Raman spectroscopy, synchrotron polarized far-infrared spectroscopy, and polarized visible light absorption spectroscopy. A series of pressure-induced structure changes are identified, which are demonstrated to be closely correlated with the evolution of octahedral distortions. The initial ferroelectric phase transforms into an antiferroelectric phase within 2–8 GPa, remains in the antiferroelectric phase between 8 and 16 GPa, and further undergoes a transition to the paraelectric phase in the 16–21 GPa range. Meanwhile, the [NbO2Cl4] octahedral ordering undergoes a transition from a parallel to an antiparallel configuration, followed by a gradual reduction in octahedral distortion. A dramatic collapse of the optical bandgap is observed at pressures above 38 GPa, suggesting that the octahedral distortion may have been suppressed. This study provides an in-depth understanding of the anisotropic optical properties of NbOCl2 and offers important implications for the design and applications of future anisotropic optoelectronic devices based on this material.