Microstructure and ordering of iron vacancies in the superconductor system KyFexSe2 as seen via transmission electron microscopy

Structural investigations by means of transmission electron microscopy (TEM) on K0.8FexSe2 and KFexSe2, 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 K0.8FexSe2 and KFexSe2 (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.