Anomalous Eu Valence State and Superconductivity in Undoped Eu_3Bi_2S_4F_4

摘要

We have synthesized a novel europium bismuth sulfofluoride, Eu_3Bi_2S_4F_4, by solid-state reactions in sealed evacuated quartz ampules. 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 CaF_2-type Eu_3F_4 layers and NaCl-like BiS_2 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 Moessbauer and magnetization measurements. The Eu~(3+) components donate electrons into the conduction bands that are mainly composed of Bi 6p_x and 6p_y states. Consequently, the material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping.