Microstructure and ordering of iron vacancies in the superconductor system K_yFe_xSe₂ as seen via transmission electron microscopy

Structural investigations by means of transmission electron microscopy (TEM) on K_0.8Fe_xSe₂ and KFe_xSe ₂, with 1.5 ≤ x ≤ 1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5 ≤ x ≤ 1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of K₂Fe₄Se₅. The superconducting K_0.8Fe_xSe₂ and KFe _xSe₂ (1.7 ≤ x ≤ 1.8) materials contain clear phase separation, in particular, along the c-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state.