First-order structural transition and pressure-induced lattice/phonon anomalies in Sr2IrO4
Abstract
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 40 and 55 GPa. At lower pressures, lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of P1=17 GPa is associated with: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal c/a ratio take place above P1; (ii) a four-fold symmetry-breaking lattice strain associated with lattice disorder; (iii) disappearance of two Raman active modes (at 180 and 260 cm-1); and (iv) development of an asymmetric Fano lineshape for the 390 cm-1 mode. DFT indicates that the phase above P1 is most likely non-magnetic. 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 P1, 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 P1 are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at P2=30 and P3=40 GPa, indicating further competing phases that are stabilized at high pressures.