Structural phase transition below 250 K in superconducting K${}_{0.75}$Fe${}_{1.75}$Se${}_2$

Abstract

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.