  We report neutron scattering and transport measurements on semiconducting
Rb$_{0.8}$Fe$_{1.5}$S$_2$, a compound isostructural and isoelectronic to the
well-studied $A_{0.8}$Fe$_{y}$Se$_2 (A=$ 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, interdigitated mesoscopically with the rhombic phase is an ubiquitous
phase with $\sqrt{5}\times\sqrt{5}$ iron vacancy order. This phase has a
magnetic transition at $T_N=425$ K and an iron vacancy order-disorder
transition at $T_{S}=600$ 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 Rb$_{0.8}$Fe$_{1.5}$S$_2$ and
K$_{0.81}$Fe$_{1.58}$Se$_2$, we argue that the in-plane antiferromagnetic order
arises from strong coupling between local moments. Superconductivity,
previously observed in the $A_{0.8}$Fe$_{y}$Se$_{2-z}$S$_z$ system, is absent
in Rb$_{0.8}$Fe$_{1.5}$S$_2$, which has a semiconducting ground state. The
implied relationship between stripe/block antiferromagnetism and
superconductivity in these materials as well as a strategy for further
investigation is discussed in this paper.
