UC Berkeley

We report neutron diffraction studies of FeS single crystals obtained from Rb _x Fe_2-y S₂ single crystals via a hydrothermal method. While no 5 × 5 iron vacancy order or block antiferromagnetic order typical of Rb _x Fe_2-y S₂ is found in our samples, we observe C-type short-range antiferromagnetic order with moments pointed along the c axis hosted by a different phase of FeS with an expanded interlayer spacing. The Néel temperature for this magnetic order is determined to be 170 ± 4 K. Our finding of a variant FeS structure hosting this C-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related FeSe_1-x S _x iron chalcogenide system.