  We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by
solid-state reactions in sealed evacuated quartz ampoules. The compound
crystallizes 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 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,
whilst Eu(2) has an average valence of +2.64(5). This anomalous Eu valence
state is further confirmed and supported, respectively, by Mossbauer and
magnetization measurements. The Eu3+ 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.
