Structural phase transition below 250 K in superconducting K0.75Fe1.75Se2

Vibrational properties of iron-chalcogenide superconductor K0.75Fe1.75Se2 with T-c similar to 30 K have been measured by Raman and optical spectroscopies over the temperature range 3-300 K. The sample undergoes I4/m -> I4 structural phase transition accompanied by loss of inversion symmetry at T-1, below 250 K, observed as the appearance of new fully symmetric Raman mode at similar to 165 cm(-1). Small vibration mode anomalies are also observed at T-2 similar to 160 K. From first-principles vibrational analysis of antiferromagnetic K0.8Fe1.6Se2 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.