Partial charge ordering in the mixed-valent compound

Crystals of the mixed-valent compound were grown from a flux. The room temperature conductivity of a crystal was 3 S/cm but decreased smoothly with decreasing temperature to 10⁻⁵ S/cm at 25 K. Magnetic susceptibility data indicate a localized moment for Rh⁴⁺. A Seebeck coefficient at 200 K of +280 μV/K further confirms that this compound is a semiconductor rather than a metal with a partially filled 4d t_2g band. A structure refinement based on single crystal X-ray diffraction data obtained at 173 and 296 K provided Rh–O distances sufficiently accurate to indicate the nature of the charge ordering between Rh³⁺ and Rh⁴⁺. The large Seebeck coefficient coupled with the high electrical conductivity indicates that this may be a promising low-temperature thermoelectric material.