title: Anomalous Eu valence state and superconductivity in undoped Eu3Bi2S4F4.
abstract: We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) Å, c = 32.4330(6) Å, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond valence sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, while Eu(2) has an average valence of ∼ +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mössbauer and magnetization measurements. The Eu(3+) components donate electrons into the conduction bands that are mainly composed of Bi 6px and 6py states. Consequently, the material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping.
authors.affiliation: Department of Physics, ¶Center of Electron Microscope, and ∥State Key Lab of Silicon Materials, Zhejiang University , Hangzhou 310027, China.
authors.forename: Hui-Fei
authors.initials: HF
authors.lastname: Zhai
authors.forename: Pan
authors.initials: P
authors.lastname: Zhang
authors.forename: Si-Qi
authors.initials: SQ
authors.lastname: Wu
authors.forename: Chao-Yang
authors.initials: CY
authors.lastname: He
authors.forename: Zhang-Tu
authors.initials: ZT
authors.lastname: Tang
authors.forename: Hao
authors.initials: H
authors.lastname: Jiang
authors.forename: Yun-Lei
authors.initials: YL
authors.lastname: Sun
authors.forename: Jin-Ke
authors.initials: JK
authors.lastname: Bao
authors.forename: Israel
authors.initials: I
authors.lastname: Nowik
authors.forename: Israel
authors.initials: I
authors.lastname: Felner
authors.forename: Yue-Wu
authors.initials: YW
authors.lastname: Zeng
authors.forename: Yu-Ke
authors.initials: YK
authors.lastname: Li
authors.forename: Xiao-Feng
authors.initials: XF
authors.lastname: Xu
authors.forename: Qian
authors.initials: Q
authors.lastname: Tao
authors.forename: Zhu-An
authors.initials: ZA
authors.lastname: Xu
authors.forename: Guang-Han
authors.initials: GH
authors.lastname: Cao