Two-dimensional (2D) magnetism is always a very broad and fundamentally interesting topic in condensed matter physics. One of the current research hotspots is to search for single-layer magnetic materials exfoliated from a bulk that possesses a robust magnetic order.^[1–5] Recent studies focus on a number of potentially 2D van der Waals (vdW) magnetic materials, including binary transition metal halides MX_y (M = metal cation; X = halogen anion),^[6,7] chromium based CrOCl,^[8–12] and transition-metal (TM) phosphorous trichalcogenides MPX₃ (M denotes TM atoms; X = S or Se).^[3,13–17] Both VI₃ and CrI₃ belong to the MX₃ trihalide family.^[18–23] CrI₃ has been extensively studied^{[1,2,24–26]} in the last few years and more and more attention is paying to VI₃ now. Generally V compounds have more complicated physical properties compared to Cr compounds. Recently single crystals of VI₃ have been grown and some basic physical properties were measured.^[27–32] However, spectroscopic studies are still highly required.

VI₃ has the layered BiI₃-type structure with VI₆ octahedra sharing edges in each layer to form a honeycomb lattice and exhibits a structural phase transition at T_s ∼ 79 K.^[27] VI₃ is a typical semiconductor and has a long-range ferromagnetic (FM) ordering at T_c ∼ 50 K.^[27] Previous studies on the crystal structure gave controversial results and no consensus has been reached yet. XRD refinements suggested two distinct high-temperature phases P31c^[29] and C2/m^[27] respectively, while ^[27] and C2/c^[29] space groups were proposed as the corresponding low-temperature phases below the structural transition. Clearly one can clarify the symmetry by distinguishing the point group. That is exactly one of the outstanding advantages Raman scattering possesses. This motivates us to perform polarized Raman scattering measurements to determine the point symmetry across the structural transition. The rich magnetism reported in VI₃ also allows us to look into the spin-related excitations/interactions by performing temperature-dependent Raman scattering experiments.

In this paper, we have performed polarized and temperature-dependent Raman scattering measurements on VI₃. The angle-dependent Raman spectra indicate that the point group C_2h remains unchanged from ∼ 10 K to 300 K, ruling out D_3d symmetry. The temperature-dependent Raman spectra of the two A_g phonon modes at 71.1 cm⁻¹ and 128.4 cm⁻¹ catch the signature of the structural phase transition. The sharp tendency of the linewidth and Fano asymmetry for both modes suggest a clear spin–phonon coupling. The abnormal behavior of 71.1-cm⁻¹ mode in frequency also confirms the spin–phonon coupling.

VI₃ single crystals used in this study were grown using the chemical vapor transport method (CVT). The details of crystal growth can be found elsewhere.^[27] We obtained the glossy crystals with flat surface and then quickly transferred the sample into a UHV cryostat with a vacuum of better than 10⁻⁸ mbar (1 bar = 10⁵ Pa). The polarized and angular Raman spectra and temperature-dependent Raman spectra were collected using a HR800 spectrometer (Jobin Yvon) equipped with a liquid-nitrogen-cooled charge-coupled device (CCD) and volume Bragg gratings, and micro-Raman backscattering configuration was adopted. A 633-nm laser was used with a spot size of ∼ 5 μm focused on sample surface. The scattered signal was collected through a 50 × long focus-length objective, and dispersed with a 600-grooves/mm grating. The laser power was maintained at approximately ∼ 1.4 mW to avoid overheating the sample during measurements. The angle dependence of mode intensities was measured by rotating light polarizations with careful adjustment of the angle matching. We define X and Y axes in the crystallographic ab plane. X is perpendicular to the b axis and Y is along the b axis. Z is perpendicular to X and Y. The incident light direction was perpendicular to the (001) plane.

Previous x-ray diffraction studies proposed two distinct high-temperature structural phases C2/m^[27] and P31c,^[29] which change to low-temperature phases ^[27] and C2/c^[29] after the structural transition, respectively. P31c belongs to D_3d point group while C2/m and C2/c share C_2h point group symmetry. The symmetry issue can be solved by determining the point group symmetry. That is exactly what Raman scattering can do.

For C_2h point group, there are 8A_g + 7B_g Raman active modes at the Γ point. The corresponding Raman tensors are

On the other hand, for D_3d point group, there are 3A_1g + 5E_g Raman active modes at the Γ point. The corresponding Raman tensors are

In Fig. 1 we show Raman spectra of VI₃ at 10 K and 300 K. There are two well-defined modes located at 71.1 cm⁻¹ and 128.4 cm⁻¹ at 300 K. The two modes only appear in parallel polarization and can be assigned to A_g modes according to the symmetry analyses above. The emergence of the 98.0-cm⁻¹ phonon mode at 10 K is an indication of thermal broadening reduction at low temperatures, rather than a phase transition. This is confirmed by its continuous evolution in intensity from 10 K to 300 K. The optical modes in CrI₃ have been calculated.^[33,34] VI₃ has exactly the same crystal symmetry with CrI₃ and hence the 98-cm⁻¹ mode in VI₃ can be safely attributed to the mode ∼ 101.9 cm⁻¹ in CrI₃ based on the energy and symmetry. The A_g mode in both VI₃ and CrI₃ represents the joint vibrations of magnetic ions and I⁻ anions.

Fig. 1.

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	Fig. 1. Polarized Raman spectra in VI3 at 300 K under (red line) and (pink line) scattering geometries. The phonon modes at 71.1 cm−1 and 128.4 cm−1 are Ag ones. Unpolarized Raman spectrum at 10 K is also shown here.

To look into the crystal symmetry at 10 K and 300 K, we analyze the angular dependence of the intensities of the two A_g modes at 71.1 cm⁻¹ and 128.4 cm⁻¹. In Figs. 2(b) and 2(c), we track the angular dependence of their intensities under cross polarizations at 10 K and 300 K. In cross channel at 300 K, the intensities of the two modes are non-zero and the crystal exhibits a four-fold symmetry. This tells us that the crystal symmetry of VI₃ at 300 K is C_2h point group if we check it with Eq. (2). Similarly, the two modes are still non-zero and the four-fold symmetry remains at 10 K. The small deviation between the experimental data and the fitting curve results from the vibration of the sample stage at low temperatures and has little effect on symmetry analysis. As the VI₆ octahedra experience an off-center distortion with the low temperature which will decrease the symmetry of crystal, the single crystal still keeps the symmetry of C_2h point group at 10 K rather than with a higher symmetry. The interlayer coupling in VI₃ is much stronger than that in the MPX₃ family. This implies that the bulk VI₃ at room temperature may not be simply described using D_3d point group.^[35]

Fig. 2.

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Fig. 2. (a) Raman spectra in cross polarization configuration (ei ⊥ es) collected by rotating light polarizations at 10 K and 300 K. (b) and (c) Intensity plots with respect to the rotation angle for Ag modes in cross channel () at 10 K and 300 K, respectively. The red (black) dots are the experimental data of 128.4 cm−1 (71.1 cm−1) modes and the solid curves are fitting curves using the intensity functions (2). The ei and es represent the polarizations of the incident and scattered light, respectively.

We further performed temperature-dependent Raman measurements to study the structural phase transition and magnetic properties in VI₃. In general, for anharmonic phonon modes, the phonon frequencies ω and the linewidths Γ as a function of temperature are given by^[36]

The frequency shift ω and the linewidth Γ of 128.4-cm⁻¹ A_g mode as a function of temperature are extracted from Fig. 3(a) and shown in Figs. 3(b) and 3(c), respectively. It can be seen that there is a frequency turn up accompanied by a drop linewidth, near structural phase transition. The anharmonic fitting is applied to the temperature ranges above and below T_s, respectively. Clearly the anomalies are caused by the structural phase transition. Besides the structural transition, it is also reported that VI₃ has an FM phase transition at ∼ 50 K.

Fig. 3.

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	Fig. 3. (a) Temperature dependence of 128.4-cm−1 Ag mode. Temperature dependence of frequency shift (b) and linewidth (c) of the Ag mode (left axis). The solid red lines in panel (b) represent the fits with Eq. (3) and the lines in panel (c) represent the fits with Eq. (4).

An interesting observation is that the two A_g modes at ∼ 71.1 cm⁻¹ and 128.4 cm⁻¹, display a pronounced Fano asymmetry at low temperatures (T < T_c) and vanishes for T > T_c, as shown in Figs. 3(a) and 4(a). As a semiconductor with a bandgap of ∼ 0.9 eV,^[27] the electron–phonon coupling effect is expected to be small. The Fano asymmetry may stem from the spin–phonon coupling.

Fig. 4.

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	Fig. 4. (a) Temperature dependence of Raman spectra of 71.1-cm−1 Ag mode. Temperature dependence of frequency shift (b) and linewidth (c) (left axis). The solid red lines in panels (b) and (c) represent the fits with Eq. (3) and Eq. (4).

Figures 4(b) and 4(c) show the frequency shift ω and the linewidth Γ of 71.1 cm⁻¹ as a function of temperature. There is also a turn point for the mode. The frequencies and linewiths have a small anomaly near T_s ∼ 79 K, which is considered to be a signature of structural phase transition. The rapid narrowing in phonon linewidth becomes below T_c ∼ 50 K, indicates that spin–phonon coupling exist in VI₃. In addition, 71.1-cm⁻¹ mode exhibits an unusual softening above the magnetic transition and goes back to the normal anharmonic behavior below the magnetic transition. This implies that 71.1-cm⁻¹ mode has a stronger spin-coupling than 128.4-cm⁻¹ phonon. The frequency of A_g phonon at 71.1 cm⁻¹ is lower, which is dominated by the vibration of magnetic ions V³⁺. It may explain why the phonon softening caused by the spin–phonon coupling in 71.1-cm⁻¹ phonon is more significant than that in 128.4-cm⁻¹ phonon. The spin–phonon coupling existing in such a large temperature range in VI₃ will be essential for wide applications.

In summary, we have carried out angle- and temperature-dependent Raman scattering measurements on VI₃ single crystals. The polarized Raman spectra indicate that the crystal symmetry at 10 K and 300 K can be described by C_2h rather than D_3d point group. The signatures of the structural phase transition are observed in the temperature-dependent Raman spectra. And interestingly, the clear asymmetry of the two A_g modes and the softening of the 71.1-cm⁻¹ mode evidence a significant spin–phonon coupling in the system. These observations clarify the symmetry issue on the structural transition and reveal the spin–phonon coupling which may be of interest for potential applications.