Neutron Scattering Study on the New 245 Iron Selenide Superconductors

We determine using neutron and x-ray diffraction method the sample composition, crystalline structure and magnetic order of the recently discovered $A_2$Fe$_4$Se$_5$ superconductors ($A$=K, Rb, Cs, Tl/K or Tl/Rb). Contrary to initial belief that these materials are heavily electron-doped variety of the BaFe2As2 family of Fe-based superconductors, they are almost charge balanced with the Fe valence close to 2+ as in the 11 iron selenide superconductors, and crystalize in an Fe vacancy-ordered lattice structure [1,2]. Coexisting with superconductivity is a novel block antiferromagnetic order which conforms to the tetragonal crystalline symmetry and possesses a very large ordered magnetic moment 3.3$\mu_B$ per Fe and a very high ordering temperature above 500 K [1]. Such Fe vacancy ordered crystal structure and coexisting antiferromagnetism and superconductivity occur in all 5 types of new iron selenide superconductors discovered so far. With Fe vacancy number departs from the chemical formulas $A_2$Fe$_4$Se$_5$, an imperfect version of the Fe vacancy order results at base temperature while phase separation into two vacancy-ordered phases exists at the intermediate temperature range [4]. The Fe site disorder renders the materials insulating and destroys the superconductivity as spin-glass disorder does in previous 11 iron selenide superconductors [5]. \\[4pt] [1] W. Bao et al., Chin. Phys. Lett. {\bf 28}, 086104 (2011).\\[0pt] [2] P. Zavalij etal., Phys. Rev. B {\bf 83}, 132509 (2011).\\[0pt] [3] F. Ye et al., Phys. Rev. Lett. {\bf 107} 137003 (2011).\\[0pt] [4] W. Bao et al., arXiv: 1102.3674 (2011).\\[0pt] [5] T.J. Liu et al., Nat. Materials {\bf 9}, 716 (2010).