Raman Scattering Investigation of Structural Transition in Ca<sub>5</sub>Ir<sub>3</sub>O<sub>12</sub>
Takumi Hasegawa, Wataru Yoshida, Kazuma Nakamura, Norio Ogita, Kazuyuki Matsuhira
Abstract
We report a study of the second-order phase transition at 105 K in the geometrically frustrated iridate Ca5Ir3O12 using a Raman scattering method. The Raman scattering spectra of a single crystal were measured from 4 K to room temperature. Ab initio phonon calculations that consider the spin–orbit interaction were also conducted and compared with the experimental spectra. Agreement between the theoretical and experimental results at room temperature is reasonably good. At room temperature, \(6\text{A'}_{1} + 9\text{E'} + 5\text{E''}\) were assigned among the Raman active modes, \(6\text{A'}_{1} + 13\text{E'} + 6\text{E''}\), based on the reported \(P\bar{6}2m\) crystal structure. Below Ts, 23 additional peaks were observed, suggesting the appearance of a superlattice structure. The polarization dependence of Raman spectra below Ts indicates the existence of \(\bar{6}\) symmetry. We observed at least one additional mode as a broad weak-intensity peak at temperatures higher than Ts. This suggests possible local distortion around the Ir ions, which would be expected for Ir ions with mixed valence states.