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Vibrational properties of iron-chalcogenide superconductorK$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}\sim$ 30 K have been measured by Ramanand optical spectroscopies over temperature range of 3-300 K. Sample undergoes\textit{I4/m} $\to $ \textit{I4} structural phase transition accompanied byloss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance ofnew fully-symmetric Raman mode at $\sim$ 165 cm$^{-1}$. Small vibration modeanomalies are also observed at $T_{2}\sim$ 160 K. From first-principlesvibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizingpseudopotentials all observed Raman and infrared modes have been assigned andthe displacement patterns of the new Raman mode identified as involvingpredominantly the Se atoms.

Structural phase transition below 250 K in superconducting K$_{0.75}$Fe$_{1.75}$Se$_{2}$