title: Understanding the anomalous behavior of Vegard's law in Ce1-xMxO2 (M = Sn and Ti; 0 < x ≤ 0.5) solid solutions.
abstract: The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x ∼ 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 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, (119)Sn Mössbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 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(4+) and Sn(4+). Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 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% 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 Ce1-xSnxO2 solid solutions.
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India and Paul Scherrer Institut, 5253 Villigen, Switzerland.
authors.forename: Tinku
authors.initials: T
authors.lastname: Baidya
authors.affiliation: Surface Engineering Division, CSIR - National Aerospace Laboratories, Bangalore 560017, India. partho@nal.res.in.
authors.forename: Parthasarathi
authors.initials: P
authors.lastname: Bera
authors.affiliation: Paul Scherrer Institut, 5253 Villigen, Switzerland and Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
authors.forename: Oliver
authors.initials: O
authors.lastname: Kröcher
authors.affiliation: Paul Scherrer Institut, 5253 Villigen, Switzerland.
authors.forename: Olga
authors.initials: O
authors.lastname: Safonova
authors.affiliation: SNBL, European Synchrotron Radiation Facility, 38043 Grenoble, France.
authors.forename: Paula M
authors.initials: PM
authors.lastname: Abdala
authors.affiliation: Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany.
authors.forename: Birgit
authors.initials: B
authors.lastname: Gerke
authors.affiliation: Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Pöttgen
authors.affiliation: Department of Physics, Goa University, Taleigao Plateau, Goa 403206, India.
authors.forename: Kaustubh R
authors.initials: KR
authors.lastname: Priolkar
authors.affiliation: Department of Chemistry and Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India.
authors.forename: Tapas Kumar
authors.initials: TK
authors.lastname: Mandal