Vacancy ordering and phonon spectrum of the iron-based superconductor K0.8Fe1.6Se2

We have performed Raman-scattering measurements on a high-quality single crystal of the recently discovered Fe-based superconductor K_0.8Fe_1.6Se₂ (T_c = 32 K). At least 13 phonon modes were observed in the wave number range 10-300 cm^-1. The spectra possess a fourfold symmetry indicative of bulk vacancy order in the Fe-deficient planes. We perform a vibration analysis based on first-principles calculations, which both confirms the ordered structure and allows a complete mode assignment. We observe an anomaly at T_c in the 180 cm^-1 A_g mode, which indicates a rather specific type of electron-phonon coupling.