Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4

Abstract

We have synthesized a novel europium bismuth sulfofluoride, Eu 3 Bi 2 S 4 F 4 , by solid-state reactions in sealed evacuated quartz ampoules. 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 CaF 2 -type Eu 3 F 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