Inelastic X-ray Scattering Study of the Hidden Order at 105 K in Ca5Ir3O12

Ca5Ir3O12 has a hexagonal structure with noncentrosymmetric space group of ?-62? (No. 189). In the structure, 1D chains of the edge-sharing IrO6 octahedra form triangular lattices in the c-plane. The average valence of Ir ions in Ca5Ir3O12 is +4.67, so Ir4+ and Ir5+ exist in a ratio of 1:2. This situation can lead to the geometrical frustration of charge on both the triangular lattice in c-plane and 1D chains along the c-axis [1]. It is reported that Ca5Ir3O12 exhibits an antiferromagnetic below 7.8 K and a second-order phase transition at 105 K [1]. The origin of this phase transition at 105 K is not clear at present; this is "hidden order" [1]. Recently, we performed Raman scattering experiments on the single crystal, and confirmed the change of Raman spectra below 105 K, which indicate the change of the crystal structure [3].
In this study, we performed inelastic X-ray scattering experiments at SPring-8 in Japan to investigate the details of the vibration mode [4,5]. We compared the experimental results with ab initio phonon dispersion considering the spin-orbit interaction. The calculation values considering SOI are in good agreement with the experimental results [4]. The obtained phonon spectra show almost no temperature change; there are no soft phonon modes. On the other hand, the superlattice reflections at q = (1/3, 1/3, 1/3) are observed below 105 K (Fig. 1) [5]. We will report these results in this presentation.