Terahertz Spectroscopy and DFT Analysis of Phonon Dynamics of the Layered Van der Waals Semiconductor Nb 3 X 8 ( X = Cl, I).

We have conducted a terahertz spectroscopic study and a density functional theory analysis of the phonon dynamics of the layered van der Waals semiconductors Nb 3 Cl 8 and Nb 3 I 8 . Several infrared-active phonon modes were observed in the terahertz region, and their frequencies were found to be in excellent agreement with our first-principles lattice dynamics calculations. For Nb 3 Cl 8 , the observed phonon spectra are consistent with a structural transition at 90 K from the high-temperature P 3̅ m 1 phase to the low-temperature R 3̅ m phase. Also, our study confirmed that the structural and magnetic transitions were coupled in Nb 3 Cl 8 . For Nb 3 I 8 , which is nonmagnetic at and below room temperature, no significant temperature or magnetic field dependence was observed in the phonon spectra. Our study provides an intriguing connection between the structural properties and the paramagnetic-nonmagnetic transitions in Nb 3 Cl 8 and Nb 3 I 8 .