Influence of the mixed-valence state of Ir on the structural properties of Ba${}_3$MIr${}_2$O${}_9$ (M = In, Lu, Mg)

Mixed-valence iridates are a new platform for frustrated magnetism as shown in T. Dey, Phys. Rev. B 96 (2017). It is suspected that the mixed-valence state may evolve with temperature. Here we used X-ray diffraction (using radiation from a synchrotron) and neutron powder diffraction in a broad temperature range to compare the mixed-valence In and Lu compounds of the Ba${}_3$MIr${}_2$O${}_9$ family of hexagonal perovskites with the integer-valence Mg compound of the same family. The temperature dependence of the structural parameters, including lattice constants and Ir-Ir distances, will be reported. Additionally the instability toward a monoclinic distortion will be discussed.