Oxygen-release/storage properties of Ce0.5M0.5O2 (M = Zr, Hf) oxides: interplay of crystal chemistry and electronic structure

Fluorite-type Ce0.5Zr0.5O2 and Ce0.5Hf0.5O2 have been synthesized by a solution combustion route, and their oxygen release and reduction have been investigated up to 850 degrees C. On reduction, the zirconium system forms two pyrochlore phases, Ce2Zr2O7 (pyrochlore-I) and Ce2Zr2O6.2 (pyrochlore-II), while the hafnium system forms only a disordered fluorite phase with the composition Ce0.5Hf0.5O1.77, under the same experimental conditions. The crystal structures of the reduction products have been characterized by powder X-ray diffraction and Rietveld refinement, and their electronic structures have been investigated by photoelectron spectroscopy and electrical conductivity measurements. Pyrochlore-I (a = 10.6727(4) A) is a semiconductor, while pyrochlore-II (a = 10.6463(8) A) is a good conductor (with a nearly temperature independent resistivity of approximately 2.5 ohm.cm in the 400-1000 K range). X-ray photoelectron spectroscopy (XPS) shows an admixture of Ce(5d,6s) with Zr(4d) and O(2p) and a significant density of states near EF in the highly reduced pyrochlore-II phase. The changes have been rationalized in terms of a qualitative energy band scheme that brings out the special role of zirconium vis-à-vis hafnium in the reduction/oxygen release properties of Ce0.5Zr0.5O2 and Ce0.5Hf0.5O2.