The role of the nominal iron content in the structural, compositional and physical properties of BaFe$_{2+{\delta}}$S$_3$

TL;DR

This study systematically investigates how nominal iron excess in BaFe$_{2+{ extdelta}}$S$_3$ influences its structural, magnetic, and transport properties, revealing that Fe off-stoichiometry has limited impact on the main phase but affects secondary phases and magnetic behavior.

Contribution

It provides a comprehensive analysis of the effects of Fe excess on BaFe$_{2+{ extdelta}}$S$_3$, clarifying the role of off-stoichiometry and offering a method to produce phase-pure crystals.

Findings

No systematic change in main phase structure with Fe excess.

Magnetic susceptibility affected by secondary phases.

Crystals without ferromagnetic impurities can be made via remelting.

Abstract

BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic and transport properties. Here, we present a systematic study of compositional, structural and physical properties of BaFe$_{2+\delta}$S$_3$ as a function of the nominal Fe excess $\delta$. As $\delta$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. The previously reported maximum of $T_N$ at $\delta$=0.1 was not confirmed. Samples with nominal $\delta$=0 present the lowest $T_N$ and the resistivity anomaly at the highest temperature $T^*$ while, for $\delta \geq 0.05$, both quantities and the transport gap are seemingly $\delta$-independent. Finally, we show that crystals free of ferromagnetic spurious phases can be obtained by remelting samples with nominal $\delta$=0.05 in a Bridgman process.