Structural phase transitions and Raman identifications of the layered van der Waals magnet ${\mathrm{CrI}}_2$

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.

Figure 1

Top and side views of the crystal structure of $M$-phase (a)–(b) and $O$-phase ${\mathrm{CrI}}_2$ (d)–(e), and the light-blue shaded part in (b) represents the geometry of $\mathrm{Cr}\text{−}{\mathrm{I}}_6$ with distorted octahedral coordination. Calculated magnetic ground states of the Cr atoms in $M$-phase (c) and $O$-phase ${\mathrm{CrI}}_2$ (f). The unit cells are marked by black dashed lines. Cr and I atoms are shown in light-blue (dark-blue) and yellow (light-purple) balls, respectively. Red and purple arrows indicate the directions of magnetic moments in magnetic ground states.

Figure 2

(a), (b) Energy barriers and structural evolutions of the phase transitions between $O$-phase and $M$-phase ${\mathrm{CrI}}_2$. The green line arrow indicates the movement direction of I atomic layer, and the green hollow arrow indicates the collective movement direction of ${\mathrm{CrI}}_2$ TL.

Figure 3

(a) Ferroelectric switching pathway of $O$-phase ${\mathrm{CrI}}_2$. The green hollow arrow indicates the collective movement direction of ${\mathrm{CrI}}_2$ TL. The black solid arrow denotes the direction of polarization. The red dotted box marks the in-plane displacement corresponding to polarization arrow. (b), (c) Phonon spectra of $O$-phase and TS phase of ${\mathrm{CrI}}_2$.

Figure 4

The calculated Raman spectra of $M$-phase (a) and $O$-phase (b) ${\mathrm{CrI}}_2$ in the antiferromagnetic ground states. The group representations and atomic motions of intense Raman-active modes are also shown. The black number in the unit cell represent the Raman frequency (in $\mathrm{c}{\mathrm{m}}^{–1}$) of the vibration mode.