Structural phase transitions and Raman identifications of the layered van der Waals magnet CrI2

Understanding the phase-transition processes of van der Waals (vdW) materials plays a vital role in their performance control and application development. Here, we investigate the structural transitions and Raman identifications of the monoclinic and orthorhombic phases of the layered vdW magnet ${\mathrm{CrI}}_{2}$. At present, the characterization of these two phases remains a challenge, and our first-principles calculations show that they each have a specific Raman spectrum, which proposes a reliable method to identify them. Moreover, we discover that ${\mathrm{CrI}}_{2}$ with orthorhombic phase exhibits both ferroelectricity and magnetism simultaneously due to its unique interlayer stacking order, indicating that it will provide a platform for the popular sliding ferroelectricity and multiferroicity.