First-order structural transition and pressure-induced lattice/phonon anomalies in Sr2IrO4

Two intriguing unresolved issues of iridate physics are the avoided metallization under applied pressure of undoped Sr2IrO4 and related materials, and the apparent absence of superconductivity under electron doping despite the similarity of the fermiology of these materials with respect to cuprates. Here, we investigate the crystal structure and lattice vibrations of Sr2IrO4 by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to 66 GPa and room temperature. Density functional theory (DFT) and ab initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an 8% collapse of the c-axis is observed at high pressures, with phase coexistence being observed between similar to 40 and 55 GPa. At lower pressures and still within the high-symmetry tetragonal phase, a number of lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of P-1 = 17 GPa is associated with the following anomalies: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal c/a ratio take place above P-1 ; (ii) a fourfold symmetry-breaking lattice strain associated with lattice disorder is observed above P-1 ; (iii) two strong Raman-active modes at ambient conditions (at similar to 180 and similar to 260 cm -1 ) are washed out at P-1 ; and (iv) an asymmetric Fano line shape is observed for the similar to 390 cm(-1) mode above P i , revealing a coupling of this phonon with electronic excitations. DFT indicates that the Ir-4(+) in-plane canted magnetic moment is unstable against volumetric compression, indicating that the phase above P-1 is most likely nonmagnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at P-1 , providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at P-1 are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at P-2 = 30 and P-3 = 40 GPa, indicating that other competing phases are favored at high pressures.