Phase transition, superstructure and physical properties of K ₂Fe ₄Se ₅

The structural features and physical properties of the antiferromagnetic K _0.8Fe _1.6Se ₂ (so-called K ₂Fe ₄Se ₅ phase) have been studied in the temperature range from 300 K up to 600 K. Resistivity measurements on both single crystal and polycrystalline samples reveal a semiconducting behavior. Structural investigations of K ₂Fe ₄Se ₅ by means of transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) demonstrate the presence of a well-defined superstructure within the a-b plane originating from a Fe-vacancy order along the [1-10] direction (indexed based on the supercell with space group of I4/m). Moreover, in situ heating structural analysis shows that K _0.8Fe _1.6Se ₂ undergoes a transition of the Fe-vacancy order to disorder at about 600 K. The phase separation and the Fe-vacancy ordering in the superconducting materials of K _xFe _2-ySe ₂ (0.2 ≤y ≤ 0.3) has been briefly discussed.