Spectroscopic insight into the interplay between structural disorder and oxidation degree in melt‐grown Ce0.5Zr0.5O2‐y compounds

Abstract

The oxygen storage capacity of CexZr1‐xO2‐y compounds arises from the mixed‐valent character of cerium, which makes the system sensitive to external factors such as temperature and oxygen partial pressure through their influence in the Ce valence, cation ordering and oxygen content. In this work, we analyze the phase relations in Ce0.5Zr0.5O2‐y compounds fabricated by the laser floating zone technique in either oxidising or reducing atmosphere. Chemical, structural, and microstructural properties are studied by X‐ray diffraction, thermogravimetry, scanning electron microscopy (energy dispersive X‐ray spectroscopy and electron backscatter diffraction), and Raman spectroscopy. The latter proves to be determinant to discriminate between phases arising from subtle variations of atomic distribution that are hardly distinguished by X‐ray diffraction or energy dispersive X‐ray analyses. Cation disordered, fluorite‐like phases are obtained when processing is performed in air, whereas cation‐ordered, pyrochlore‐related phases are formed when processing in Ar+5%H2 atmosphere. Processing in a low but non‐negligible pO2 yields, besides the pyrochlore and fluorite‐like phases, an intermediate state with partial disorder that may be relevant for catalysis applications. The vibrational Raman spectrum of the Ce2Zr2O7 pyrochlore has been fully interpreted through polarisation measurements at room and low temperature. Bands arising from crystal field transitions within the ground and first excited multiplets of Ce3+ ions are also identified. The electronic Raman spectrum is found to be a sensitive probe of the degree of cation and oxygen order in these compounds.