Spin state of Co$^{3+}$ in LaCo$_{1-x}$Rh$_{x}$O$_{3}$ investigated by structural phenomena

Abstract: Neutron diffraction for a polycrystalline sample of LaCo${0.8}$Rh${0.2}$O${3}$ and synchrotron x-ray diffraction for polycrystalline samples of LaCo${0.9}$Rh${0.1}$O${3}$ and LaCo${0.8}$Rh${0.2}$O${3}$ have been carried out in order to investigate the structural properties related with the spin state of Co$^{3+}$ ions. We have found that the values of the Co(Rh)-O bond lengths in the Co(Rh)O${6}$ octahedron of LaCo${0.8}$Rh${0.2}$O${3}$ are nearly identical at 10 K. The lattice volume for the Rh$^{3+}$ substituted samples decreases with the thermal expansion coefficient similar to that of LaCoO${3}$ from room temperature, and ceases to decrease around 70 K. These experimental results favor a mixed state consisting of the high-spin-state and low-spin-state Co$^{3+}$ ions, and suggest that the high-spin-state Co$^{3+}$ ions are thermally excited in addition to those pinned by the substituted Rh$^{3+}$ ions.