Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

Abstract

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering. The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab initio lattice dynamical calculations based on nonmagnetic density-functional-perturbation theory. Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of Sr2IrO4. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in Sr3Ir2O7 , which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.