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We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, bysolid-state reactions in sealed evacuated quartz ampoules. The compoundcrystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) A, c =32.4330(6) A, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2bilayers stack alternately along the crystallographic c axis. There are twocrystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoffpositions 4e and 2a, respectively. Our bond-valence-sum calculation, based onthe refined structural data, indicates that Eu(1) is essentially divalent,whilst Eu(2) has an average valence of +2.64(5). This anomalous Eu valencestate is further confirmed and supported, respectively, by Mossbauer andmagnetization measurements. The Eu3+ components donate electrons into theconduction bands that are mainly composed of Bi- 6px and 6py states.Consequently, the material itself shows metallic conduction, and superconductsat 1.5 K without extrinsic chemical doping.

Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4