Crystal structure of Ca₅(Sc_0.4Ti_0.6)₄Fe_2As2O11

Single crystals of Ca-5(Sc0.4Ti0.6)(4)Fe2As2O11 exhibiting highly anisotropic properties compared to other iron-based superconducting materials were prepared by the self-flux method. Fundamental X-ray diffraction spots from a single crystal were indexed with tetragonal cell parameters of a = 3.8964(1) angstrom, c = 41.2890(12) angstrom, and weak diffuse streaks were observed along the c* direction at a period of a*/2. The fundamental diffraction data were analyzed using an average crystal structure model (space group I4/mmm). [FeAs] layers and perovskite-type block layers alternate in the crystal structure. Each block layer is composed of four sublayers: a central structure consisting of two individual perovskite-type [Ca-8(Sc/Ti)O-6] sublayers, and two outer oxygen-deficient [Ca-8(Sc/Ti)O-5] sublayers. In the [Ca-8(Sc/Ti)O-6] sublayers, the oxygen positions are split into four with equal probability. The chemical composition was determined to be Ca-5(Sc0.4Ti0.6)(4)(FeAs)(2)O-11, and the Ti content of greater than 0.5 suggests that the superconductivity can be attributed to electron doping.