title: Percolating hierarchical defect structures drive phase transformation in Ce1-x Gd x O2-x/2: a total scattering study.
abstract: A new hierarchical approach is presented for elucidating the structural disorder in Ce1-x Gd x O2-x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd-Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials.
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy ; Istituto di Scienze e Tecnologie Molecolari, CNR-ISTM, Milano I-20133, Italy.
authors: Marco
authors: M
authors: Scavini
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy ; Istituto per l'Energia e le Interfasi, CNR-IENI, C.so Promessi Sposi 29, Lecco I-23900, Italy.
authors: Mauro
authors: M
authors: Coduri
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy.
authors: Mattia
authors: M
authors: Allieta
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy.
authors: Paolo
authors: P
authors: Masala
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy.
authors: Serena
authors: S
authors: Cappelli
authors: Dipartimento di Chimica, Università di Milano, via C. Golgi 19, Milano I-20133, Italy ; Istituto di Scienze e Tecnologie Molecolari, CNR-ISTM, Milano I-20133, Italy.
authors: Cesare
authors: C
authors: Oliva
authors: SNBL/ESRF, 71 Avenue des Martyrs, CS 40220, Grenoble Cedex 9, 38043, France.
authors: Michela
authors: M
authors: Brunelli
authors: Dipartimento di Fisica, Università di Milano, Via G. Celoria 19, Milano I-20133, Italy.
authors: Francesco
authors: F
authors: Orsini
authors: ESRF - The European Synchrotron, 71 Avenue des Martyrs, CS 40220, Grenoble Cedex 9, 38043, France.
authors: Claudio
authors: C
authors: Ferrero
keywords: disorder; doped ceria; electron spin resonance; hierarchy; high-resolution X-ray powder diffraction; pair distribution function; percolation; solid electrolytes