Structural phase transition below 250 K in superconducting   K$_{0.75}$Fe$_{1.75}$Se$_{2}$
  Vibrational properties of iron-chalcogenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}\sim$ 30 K have been measured by Raman and optical spectroscopies over temperature range of 3-300 K. Sample undergoes \textit{I4/m} $\to $ \textit{I4} structural phase transition accompanied by loss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance of new fully-symmetric Raman mode at $\sim$ 165 cm$^{-1}$. Small vibration mode anomalies are also observed at $T_{2}\sim$ 160 K. From first-principles vibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizing pseudopotentials all observed Raman and infrared modes have been assigned and the displacement patterns of the new Raman mode identified as involving predominantly the Se atoms. 
A. Ignatov (1), A. Kumar (1), P. Lubik (1), R. H. Yuan (2), W. T. Guo   (2), N. L. Wang (2), K. Rabe (1) and G. Blumberg (2)...
cond-mat.supr-con