Pressure-Induced Structural Transitions in the Bulk van der Waals Magnet CrSBr using Synchrotron Infrared, Raman Spectroscopy, and Group-Subgroup Analysis

This study presents the first spectroscopic analysis of pressure-induced changes in the vibrational properties of CrSBr, a layered van der Waals material. Using Raman and synchrotron infrared spectroscopy, we track the evolution of vibrational phonon modes across a range of pressures, revealing phase transitions at critical pressures of 7.6, 15.3, and 20.2 GPa. The observed changes in the vibrational spectrum indicate phase transitions at these pressures, accompanied by complex modifications in the material’s symmetry. The identified phase transitions correlate with the appearance and disappearance of phonon modes (along with other factors), suggesting dynamic symmetry alterations in CrSBr’s structure. This research proposes that CrSBr simultaneously decreases and increases in symmetry along the proposed space group pathway of P mmn → P 21/m → P 2/m across the first critical pressure at 7.6 GPa. The symmetry analysis, grounded in group-subgroup relations - employing extensive use of the Bilbao Crystallographic Server - shines light upon the behavior of CrSBr’s first phase transition. The second phase transition at 15.3 GPa is due to the buckling of pendant halide atoms as the van der Waals gap decreases. This is similar to other transition metal chalcohalides such as CrOCl and FeOCl found in the literature. This research provides insights into the high-pressure behavior of this two-dimensional material and its subsequent structural changes.