Superconductivity and magnetism in Rb _0.8Fe _1.6Se ₂ under pressure

High-pressure magnetization, structural, and 57Fe Mössbauer studies were performed on superconducting Rb _0.8Fe _1.6Se _2.0 with T _c = 32.4 K. The gradual decrease of T _c with increasing pressure up to 5 GPa is followed by a step-like suppression of superconductivity at higher pressures. No structural phase transition in the Fe vacancy-ordered superstructure is observed in synchrotron x-ray diffraction (XRD) studies up to 15.6 GPa. The Mössbauer spectra above 5 GPa reveal the appearance of a new paramagnetic phase, exhibiting magnetic order below 80 K, coinciding with the irreversible disappearance of superconductivity. We interpret these changes as due to a pressure-induced diffusion of Rb ions between the two phases, responsible for the antiferromagnetism and superconductivity in Rb _0.8Fe _1.6Se _2.0.