Probing IrTe2 crystal symmetry by polarized Raman scattering

Polarized Raman scattering measurements on IrTe$_2$ single crystals carried out over 15~K - 640~K temperature range, and across the structural phase transition, reveal new insights regarding the crystal symmetry.\ In the high temperature regime three Raman active modes are observed at all studied temperatures above the structural phase transition, rather than two as predicted by the factor group analysis for the assumed $P\bar{3}m1$ symmetry. This indicates that the actual symmetry of the high temperature phase is lower than previously thought.\ Observation of an additional E$_g$ mode at high temperature can be explained by doubling of the original trigonal unit cell along the $c$-axis and within the $P\bar{3}c1$ symmetry.\ In the low temperature regime (below 245 K) the new Raman modes appear as a consequence of the symmetry lowering phase transition and corresponding increase of the primitive cell.\ All the modes observed below the phase transition temperature can be assigned within the monoclinic crystal symmetry.\ Temperature dependence of the Raman active phonons in both phases are mainly driven by anharmonicity effects. The results call for reconsideration of the crystallographic phases of IrTe$_2$.