Two spatially separated phases in semiconducting Rb0.8Fe15S2

We report neutron scattering and transport measurements on semiconducting Rb08Fe1.552, a compound isostructural and isoelectronic to the well-studied A(0.8)Fe(15)S(2) (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 root 5 x root 5 iron vacancy order. This phase has a magnetic transition at T-N = 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 Rb0.8Fe15S2 and Rb0.8Fe15S2, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the A(0.8)Fe(15)S(2) system, is absent in Rb0.8Fe15S2, 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.