Microstructure and structure phase transition in iron-based superconductors

A rich variety of structural and physical properties have been observed in the Fe-based superconductors. Structural phase transitions and the competition of multi-ordered states commonly appear in the typical Fe-based systems. For instance, LaFeAsO undergoes a structural transition at about 150K from tetragonal (P4/nmm) to the orthorhombic (Cmma) phase in association with visible anomalies in its magnetization. Structural studies on CaFe2As2 have revealed the presence of a pseudo-periodic modulation and remarkable twinning domains at low temperatures. This kind of domain results directly from the tetragonal-to-orthorhombic transition. On the other hand, the KxFe2-ySe2 （0.7≤x≤0.8, 0.2≤y≤0.4） materials often contain Fe-vacancies in the FeSe layers; it is believed that the ordered behavior of these vacancies play a critical role in our understanding of the microstructure and physical properties of this layered superconducting system.