Proximity to a commensurate charge modulation in IrTe_2-xSe_x (x = 0 and 0.45) revealed by Raman spectroscopy

Optical phonons are employed to probe a structural phase transition of IrTe_2-xSe_x at T_C ∼ 280 K for x = 0 and TC ∼ 491 K for x = 0.45, which is dictated by Ir dimerization with charge modulations. Phonon Raman spectra show a discontinuous change at TC and thermal hysteresis. A165 cm^-1 A_g mode involving out-of-plane motions of Te atoms is mostly affected by Se doping. This highlights the role of interlayer Te-Te interactions in inducing the structural instability. For temperatures below T_C both x = 0 and x = 0.45 compounds exhibit commonly five zone-folded peaks for the 128 cm^-1. E_g in-plane mode and six zone-folded peaks for the 165 cm^-1. A_g mode. This is taken as a signature of a respective quintupling and sextupling of the unit cell. Our results, on the one hand, are consistent with a 1/5 charge modulation reported by previous studies and, on the other hand, indicate that the ground state of IrTe_2-xSe_x lies in proximity to a 1/6 commensurate charge modulation phase.