Persistent insulating state at megabar pressures in strongly spin-orbit coupled S r2Ir O4

It is commonly anticipated that an insulating state will collapse in favor of an emergent metallic state at high pressures: The average electron density must increase with pressure, while the electronic bandwidth is expected to broaden and fill the insulating energy band gap. Here we report an unusually stable insulating state that persists up to at least 185 GPa in Sr2IrO4, the archetypical spin-orbit-driven Jeff=1/2 insulator. This study shows that the electrical resistance R of single-crystal Sr2IrO4 initially decreases with applied pressure P, reaches a minimum in the range 32-38 GPa, then abruptly rises to recover the insulating state with increasing P up to 185 GPa. However, evidence of a saturation of R below 80 K for P≥124GPa GPa raises the possibility of a low-temperature exotic state. Our synchrotron X-ray diffraction and Raman scattering data show the emergence of the rapid increase in R is accompanied by a structural phase transition from the native tetragonal I41/acd phase to an orthorhombic Pbca phase (with much reduced symmetry) at 40.6 GPa. The clear correspondence of the onset pressures of these two anomalies is key to understanding the stability of the insulating state at megabar pressures: Pressure-induced, structural distortions prevent the expected onset of metallization, despite the sizable volume compression attained at the highest pressure accessed in this study.