Understanding the anomalous behavior of Vegard's law in Ce(1-x)M(x)O(2) (M=Sn and Ti; 0 less than x less than or equal to 0.5) solid solutions

Abstract

The dependence of the lattice parameter on dopant concentration in Ce(1-x)M(x)O(2) (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x approx 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce(1?x)Zr(x)O(2) solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, sup(119)Sn Mossbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce(1-x)Sn(x)O(2) is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce sup(4+) and Sn sup(4+). Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce(1-x)Sn(x)O(2). In contrast, for Ce(1?x)Zr(x)O(2) solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50 percent Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce(1-x)Sn(x)O(2) solid solutions.