Revisiting the influence of Fe excess in the synthesis of BaFe2S3

BaFe2S3 is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature TN, 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% ( d = 0.1) excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether a 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 BaFe2+delta S3 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 average composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. While a small excess Fe (2.5 mol%, i.e., delta = 0.05) can slightly increase T-N and decrease the temperature of the resistivity anomaly at T *, a range of T'(N)s/T *'s is observed within each batch. This result strongly contrasts with the previously reported maximum of T-N at delta = 0.1. Rather than with the value of delta, T-N and T * seem to correlate with the broadening in the logarithmic derivative of the resistivity around T-N that could be an indicator of the level of disorder in the samples. Finally, we show that crystals free of ferromagnetic secondary phases can be obtained by remelting samples with nominal d = 0.05 in a Bridgman-like process based on optical heating.

Automated summary

BaFe2S3 is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The Fe content has been found to play a decisive role in optimizing the magnetic ordering temperature and superconducting properties. However, an excess of Fe may lead to the presence of magnetic secondary phases. Magnetic ordering temperature and resistivity anomaly temperature seem to correlate with the level of disorder in the samples rather than with the value of Fe excess delta.