Partial charge ordering in the mixed-valent compound (Bi{sub 6}O{sub 5})Rh{sub 8}{sup 3+}Rh{sub 4}{sup 4+}O{sub 24}

Crystals of the mixed-valent compound (Bi{sub 6}O{sub 5})Rh{sub 8}{sup 3+}Rh{sub 4}{sup 4+}O{sub 24} were grown from a flux. The room temperature conductivity of a crystal was 3 S/cm but decreased smoothly with decreasing temperature to 10{sup -5} S/cm at 25 K. Magnetic susceptibility data indicate a localized moment for Rh{sup 4+}. A Seebeck coefficient at 200 K of +280 {mu}V/K further confirms that this compound is a semiconductor rather than a metal with a partially filled 4d t{sub 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 Rh{sup 3+} and Rh{sup 4+}. The large Seebeck coefficient coupled with the high electrical conductivity indicates that this may be a promising low-temperature thermoelectric material. - Graphical abstract: Crystals of the mixed-valent compound (Bi{sub 6}O{sub 5})Rh{sub 8}{sup 3+}Rh{sub 4}{sup 4+}O{sub 24} show semiconducting properties, and magnetic susceptibility data indicate a localized moment for Rh{sup 4+}. Structure refinements at 173 and 296 K show partial charge ordering between Rh{sup 3+} and Rh{sup 4+}.