Raman scattering study of two-dimensional magnetic van der Waals compound VI₃

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI₃, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI₃ from 10 K to 300 K, with focus on the two A_g phonon modes at ～ 71.1 cm^-1 and 128.4 cm^-1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C_2h rather than D_3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI₃. Fano asymmetry and anomalous linewidth drop of two A_g phonon modes at low temperatures, point to a significant spin-phonon coupling. This is also supported by the softening of 71.1-cm^-1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI₃. And spin-phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.