Temperature Dependence of $\sqrt{2} \times \sqrt{2}$ Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

We report the structural phase transition properties of the newly discovered K0.8Fe1.6Se2 superconductor (T c =31.8 K) using synchrotron single-crystal X-ray diffraction. The basic structure of the sample at room temperature is found to be tetragonal ThCr2Si2-type, modulated by a vacancy ordering induced superlattice structure together with a coexisting minority phase having a √2×√2 ordering. At 520 K, the reflections corresponding to the √2×√2 phase merge with the parent tetragonal phase. The superlattice peaks corresponding to the vacancy ordering disappear at 580 K, indicating an order-disorder phase transition at this temperature.