Partial charge ordering in the mixed-valent compound (Bi6O5)Rh83+Rh44+O24

Crystals of the mixed-valent compound (Bi6O5)Rh83+Rh44+O24 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 mu V/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. (C) 2007 Elsevier Inc. All rights reserved.