Abstract
We report neutron scattering and transport measurements on semiconducting Rb0.8Fe1.5S2, a compound
isostructural and isoelectronic to the well-studied A0.8FeySe2(A = K, Rb, Cs, Tl/K) superconducting systems.
Both resistivity and dc susceptibility measurements reveal a magnetic phase transition at T = 275 K. Neutron
diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which
exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, the stripe antiferromagnetic
phase interdigitates mesoscopically with an ubiquitous phase with
√
5 ×
√
5 iron vacancy order. This phase
has a magnetic transition at TN = 425 K and an iron vacancy order-disorder transition at TS = 600 K. These
two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on
the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in
semiconducting Rb0.8Fe1.5S2 and K0.81Fe1.58Se2, we argue that the in-plane antiferromagnetic order arises from
strong coupling between local moments. Superconductivity, previously observed in the A0.8FeySe2−zSz system,
is absent in Rb0.8Fe1.5S2, which has a semiconducting ground state. The implied relationship between stripe and
block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation
is discussed in this paper.
