Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4

TL;DR

This study reports the synthesis of a novel Eu3Bi2S4F4 compound exhibiting an unusual Eu valence state, metallic conduction, and superconductivity at 1.5 K without doping, expanding understanding of Eu-based superconductors.

Contribution

The paper introduces a new Eu-based compound with an anomalous Eu valence state and demonstrates its intrinsic superconductivity at low temperature.

Findings

Eu3Bi2S4F4 exhibits an anomalous Eu valence state of +2.64.

The compound shows metallic conduction without doping.

Superconductivity occurs at 1.5 K.

Abstract

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.