Pressure-induced phase transition and phonon anomalies in Sr₂IrO₄

Full text: The combination of strong spin-orbit interaction and moderate electronic correlation makes the 5d transition metal oxide Sr₂IrO₄ a potentially interesting system for its novel and exotic spin-orbit entangled Jeff = 1/2 Mott-insulating state. The application of external pressure can tune the inter atomic distance and atomic arrangements, leading to a tuning of spin-orbit interaction, as well as the electronic and magnetic ground states. The lattice parameters and vibrations of Sr₂IrO₄ were investigated by simultaneous Raman scattering and x-ray powder diffraction experiments as a function of pressure up to 45 GPa. The XRD and Raman experiments were carried out using XDS beamline of LNLS (E = 20 KeV) and 514.5 nm laser line, respectively. The Sr₂IrO₄ belongs to a distorted tetragonal structure with space group I41/acd. The a lattice parameter compressed much faster (30%) than c-axis with pressure, indicating an increased non-cubic crystal field with increasing pressure. We have observed the anomalies of lattice parameters and line-width at two pressure regions at ~16 and 35 GPa, indicative of phase transition, without clear accompanying change in lattice symmetry. Raman scattering studies of Sr₂IrO₄ shows the disappearance of two A1g modes at 179 and 255 cm^-1 at ~17 GPa. A Fano-type asymmetric line shape of B2g mode at 386 cm^-1 was observed in the pressure between 16 to 35 GPa; this asymmetry disappear out of this pressure range. The Fanointerference is revealing that the low-energy orbital dynamics of Ir⁴⁺ (5d5) electrons is impacted by the transitions. These results reveal a competition between distinct spinorbit configurations tuned by external pressure. (author)