Study of phonon dispersion of iridium oxide Ca₅Ir₃O₁₂ with strong spin-orbit interaction

In this study, we report the results of experiments and calculations of phonon dispersion of iridium oxide with a strong spin-orbit interaction (SOI). Using inelastic X-ray scattering (IXS), we measured the IXS spectra of Ca₅Ir₃O₁₂ along Γ-A, Γ-M, and Γ-K-M directions in the Brillouin zone of a hexagonal lattice at room temperature. We also show ab initio density-functional phonon dispersions based on local density approximation and generalized gradient approximation (GGA) considering SOI. By comparing the experimental and calculated results, we found that the GGA phonon dispersion with SOI is in very good agreement with the experimental results. The phonon calculation was performed for supercells of 1 × 1 × 3 and preliminary 2 × 2 × 1. We found no phonon instability within these supercells. Low-energy phonon properties such as Debye temperature, specific heat, and sound velocity are also discussed. (author)