Raman-scattering measurements were made of a high-quality single crystal of the recently discovered Fe-based superconductor K0.8Fe1.6Se2 (Tc = 32K). At least 13 phonon modes were observed in the wave number range 10–300/cm. The spectra possess a fourfold symmetry indicative of bulk vacancy order in the Fe-deficient planes. A vibration analysis was performed which was based upon first-principles calculations. This both confirmed the ordered structure, and permitted a complete mode assignment. An anomaly at Tc was observed in the 180/cm Ag mode, which indicated a rather specific type of electron-phonon coupling.

Vacancy Ordering and Phonon Spectrum of the Iron-Based Superconductor K0.8Fe1.6Se2. A.M.Zhang, K.Liu, J.H.Xiao, J.B.He, D.M.Wang, G.F.Chen, B.Normand, Q.M.Zhang: Physical Review B, 2012, 85[2], 024518