title: New-Structure-Type Fe-Based Superconductors: CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs).
abstract: Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Iyo
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.|IMRA Material R&D Co., Ltd. , 2-1 Asahi-machi, Kariya, Aichi 448-0032, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Kawashima
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.|Department of Physics, Tokyo University of Science , 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan.
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Kinjo
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.|Department of Physics, Tokyo University of Science , 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan.
authors.forename: Taichiro
authors.initials: T
authors.lastname: Nishio
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Ishida
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujihisa
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Yoshito
authors.initials: Y
authors.lastname: Gotoh
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Kihou
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Eisaki
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST) , 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Yoshida