Observation of phase transitions in IrTe₂ using second harmonic generation

IrTe₂ exhibits a trigonal structure at room temperature and experiences a first-order structural transition, with the transition temperature determined by whether the material is cooling or warming. We employed the second harmonic generation (SHG) technique to investigate single crystals of IrTe₂ in a temperature range spanning from 7 K to 295 K. Using this method, we observed alterations in surface symmetries consistent with the structural phase shifts at approximately 270 K and 165 K, as previously reported in low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and SHG measurements conducted at near room temperature. Our SHG polarimetry measurements revealed that the (001) plane of IrTe₂ maintains a trigonal (3m) symmetry above 270 K and then undergoes a first-order phase transition to a triclinic (1) symmetry below 270 K. Nevertheless, the 3m symmetry persists until about 165 K. Notably, we observed the emergence of a 3m symmetry at 7.2 K which was not documented in prior studies at lower temperatures. This finding supports an earlier STM measurement suggesting that the hexagonal phase with the trigonal symmetry may be related to the superconducting phase.