Probing Complex Disorder in Ce_1-xGd_xO_2-x/2Using the Pair Distribution Function Analysis

Abstract

In this work the first Pair Distribution Function (PDF) study on Ce1‑x Gd x O2‑x/2 (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration x Gd (0 ≤ x Gd ≤ 1) of the CGO solid solutions was investigated through high resolution synchrotron radiation powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions the presence of positional disorder, which has been explicitly mapped into the real space. The average structural models, as obtained by the Rietveld method, fit well the experimental PDF data only for a spatial range r > ∼10 Å. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ∼6 Å range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO system. Gd-rich and Ce-rich droplets coexist in the subnanometric range.

Simulations of doped CeO2 at finite dopant concentrations

“…In Ce1-xGdxO2-x/2 (GDC), for example, extended defects are thought to be present locally 6 such that solid solutions contain small droplets of CeO2 and Gd2O3 on the sub-nm scale, 7 even though the average structure remains fluorite and percolation theory can be used to rationalise the decrease of conductivity at larger concentrations. Nevertheless, the exact nature, structure and behaviour of extended defect clusters as a function of dopant concentration remains unclear.…”

“…Calculated cation-cation radial distribution functions support the interpretation of power diffraction data using Pair Distribution Functions (rather than a more standard Rietveld analysis). 6 Several authors have postulated that the ionic conductivity is related to the trapping of vacancies around clusters of Gd 3+ defects. 35 The picture which emerges from our structural analysis which includes the examination of local cation environments using order parameters is that an alternative description is the formation of nanodomains with the Gd2O3 structure.…”

Size and spatial correlation of defective domains in yttrium-doped CeO2

“…To overcome some of those drawbacks, we recently exploited the pair distribution function (PDF) method to investigate doped ceria compounds upon doping with Gd (Scavini et al, 2012), Y (Coduri et al, 2012a(Coduri et al, , 2013a, and other rare earths (Coduri et al, 2012b(Coduri et al, , 2014, also under fuel cell operating conditions (Coduri et al, 2013b). The advantage of the PDF analysis over EXAFS lies in that the PDF function [i.e.…”

“…The PDF technique is also bulk-probing, as it relies on the scattered intensity from all the active volume of the sample. These capabilities are well suited to studying doped ceria, which brings in the so-called nanostructure problem (Billinge and Levin, 2007): the structure of concern is singlephase though perturbed on average by large, doping-induced static disorder, as inferred from the compositional evolution of atomic displacement parameters (Coduri et al, 2012a(Coduri et al, , 2012b(Coduri et al, , 2013bScavini et al, 2012).…”

“…sub-nanometre regions with a distorted fluorite (CeO 2 ) or C-type (dopant oxide) structure. The relative amount of the two kinds of droplets follows the compound stoichiometry (Coduri et al, 2012a(Coduri et al, , 2012b(Coduri et al, , 2013bScavini et al, 2012).…”