Oxygen-release/storage properties of Ce_0.5M_0.5O₂ (M= Zr, Hf) oxides: interplay of crystal chemistry and electronic structure

Abstract

Fluorite-type Ce_0.5Zr_0.5O₂ and Ce_0.5Hf_0.5O₂ have been synthesized by a solution combustion route, and their oxygen release and reduction have been investigated up to 850°C. On reduction, the zirconium system forms two pyrochlore phases, Ce₂Zr₂O₇ (pyrochlore-I) and Ce₂Zr₂O_6.2 (pyrochlore-II), while the hafnium system forms only a disordered fluorite phase with the composition Ce_0.5Hf_0.5O_1.77, 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) Å) is a semiconductor, while pyrochlore-II (a= 10.6463(8) Å) is a good conductor (with a nearly temperature independent resistivity of ~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-a-vis hafnium in the reduction/oxygen release properties of Ce_0.5Zr_0.5O₂ and Ce_0.5Hf_0.5O₂.