Observation of phase transitions in IrTe₂ using second harmonic generation

IrTe₂ forms a trigonal structure at room temperature and undergoes a first-order structure transition with the transition temperature depending on cooling or warming. We used the second harmonic generation (SHG) technique to study IrTe₂ single crystals over the temperature range of 7 K to 295 K. Using this technique, we observed changes in surface symmetries consistent with the structural phase transitions at ~270 K and ~165 K reported in previous low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and near room temperature SHG measurements. Our SHG polarimetry measurements show that the (001) plane of IrTe₂ has a trigonal (3m) symmetry above ~270 K and undergoes a first order phase transition to a triclinic (1) symmetry below ~270 K. However, the 3m symmetry does not completely vanish until ~165 K. In addition, we observed an interesting feature not reported in previous studies at lower temperatures, the recovery of the 3m symmetry at 7.2 K. This observation is supportive to an earlier STM measurement claiming that only the hexagonal phase with that trigonal symmetry is responsible for the onset of superconductivity (T_c = 2.5 K).