Two spatially separated phases in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$

TL;DR

This study reveals two distinct magnetic and structural phases in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$, highlighting the relationship between magnetic order and the absence of superconductivity in this compound.

Contribution

It identifies and characterizes two spatially separated phases with different iron vacancy orders and magnetic transitions, providing insight into the magnetic structure of semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$.

Findings

Two phases with different iron vacancy orders coexist.

Magnetic transitions at 275 K and 425 K for the two phases.

No superconductivity observed in Rb$_{0.8}$Fe$_{1.5}$S$_2$.

Abstract

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.