Crystal Structure Analysis of Nd-Doped Ceria Solid Solutions

This paper deals with Nd-doped ceria solid solutions: Ce1-xNdxO2- with "x" ranging from 0 to 0.25. Six different powders were synthesized by applying the method based on selfpropagating room temperature reaction (SPRT) between metallic nitrates and sodium hydroxide. The method is known to assure very precise stoichiometry of the final product in comparison with a tailored composition. Rietveld refinement was employed to get structural information on the synthesized powder. An increase of Nd ion concentration increases the unit cell parameters and average bond distances. We have shown that all obtained powders were solid solutions with a fluorite-type crystal structure and all powder particles were of nanometric size (about 3 nm).


Introduction
Ceria based materials have excellent oxygen storage behavior.This behavior results from the balance between reduced and oxidized states of ions i.e., Ce +3 and Ce +4 and from increased oxygen transport capacity.Thus, ceria is very promising material for use as an electrolyte in solid oxide fuel cell (SOFC) applications 1, 2.For comparable doping conditions, the overall oxygen ionic conductivity in doped ceria is approximately an order of magnitude greater than that of stabilized zirconia 3.The larger ionic radius of Ce 4+ (0.97 Å) than Zr 4+ (0.72 Å), results in a much more open structure through which oxygen ions can easily migrate 4.This allows ceria to be used as an electrolyte at moderate operating temperatures.
However, the mentioned properties are strongly dependent on the structural features.Therefore for the design of ceria based materials with high oxygen storage and transport capacity it is important to know how to increase the number of structural defects (oxygen vacancies) and to maintain at the same time a fluorite-type crystal structure.There are two possibilities to obtain ceria-based oxide as an oxygen storage component, either by promotion of Ce 4+ reduction into Ce 3+ or to chemically dope ceria with other transition or rare-earth elements 5.
The key factor in the design of modified ceria is the choice of dopant elements, as well as their introduced amounts.In addition, the preparation method of the powder has also a very strong influence on the homogeneity and stability of the solid solutions.In this work the powders were prepared by the self-propagating room temperature reaction (SPRT) 6.This technique gives the possibility of producing very fine powders with very precise stoichiometry in accordance with the tailored compositions 7.
This paper describes in addition, characterization of a number of solid solutions of neodymium doped ceria by X-ray diffraction using Rietveld refinement in order to study the variation of the lattice parameter with Nd content and microstructure size-strain behavior.

Experimental
Solid solutions of neodymium doped ceria samples were synthesized by a SPRT method using nitrates of Ce and Nd (Aldrich, USA) and NaOH (p.a.Zorka, Serbia) as starting materials.The compositions of reacting mixtures were calculated according to the nominal composition of the final reaction product.Compositions of Ce 1-x Nd x O 2- powders were synthesized with x ranging from 0 to 0.25.Preparation of Ce 1-x Nd x O 2- powders was performed according to reaction: The above reaction belongs to a group of double exchange reactions which proceeds at room temperature after the mixture of reactants was for a very short time, mechanically activated by hand mixing in mortar.The products were centrifuged in order to eliminate NaNO 3 .After drying at 60C in ambient atmosphere, the structure of the solid solutions was identified by means of X-ray powder diffraction (XRD) on a Siemens D-500 XRD diffractometer with Cu K radiation at room temperature.Data for structural refinement were taken in the 2θ range 20 -100 º, with a step width of 0.025 º and 5 s per step.The structure refinement was carried out on a Fullprof program which adopts the Rietveld calculation method.A pseudo-Voigt function was chosen as a profile function among profiles in the refinement program.Linebroadening analysis was performed using the Rietveld method in conjunction with the Warren-Averbach procedure in order to obtained the crystallite size and lattice microstrain parameters.In the present approach the grain size broadening was represented by a Lorentzian function, and microstrain broadening by a Gaussian function.The convolution of these functions is a Voigt function which is approximated by a modified Thompson-Cox-Hastings pseudo-Voigt function [8].

Results and discussion
The crystal structure of pure ceria is shown in Fig. 1.Ce occupies FCC lattice positions and O occupies all tetrahedral sites.Thus, around Ce 4+ there are eight O 2-in the nearest neighbor shell, and 12 Ce 4+ in the next nearest neighbor shell.After doping with Nd 2 O 3 , for example, some oxygen sites change to vacancy sites and some Ce 4+ are substituted by Nd 3+ causing structure distortion.The most common way to clarify the structural change is X-ray Rietveld refinement.Rietveld refinement requires a structural model that has an approximation for the actual structure.The starting structural model for the cubic system was built up with crystallographic data reported by Kuemmerle and Heger 9.
According to X-ray diffraction analysis, the obtained powders are single phase, independent of dopant concentration in the range investigated.The best fit between calculated and observed X-ray diffraction pattern is shown in Fig. 2 and Fig. 3(a-d).Peaks related to isolated Nd phases or the spurious phases are not observed.All solid solution powders exhibit the fluorite crystal structure.The dissolution of Nd 2 O 3 in a cubic fluorite lattice causes shifting of ceria peaks toward lower angles indicating the existence of a solid solution (Fig. 4).This behavior coincides with changes of lattice constant, a o (Tab.I).Namely, replacement of smaller Ce 4+ ions with larger Nd 3+ ions (the ionic radii of Ce 4+ and Nd 3+ are 0.97 and 1.1053 Å [10]) leads to the cubic ceria lattice expansion [11].The results obtained for the Ce ion occupation factor show that there is very good agreement between experimental and calculated values.For the Ce 0.75 Nd 0.25 O 2- solid solution, there is a 4% of discrepancy.It can be attributed to changes in the anion vacancy radius or to a decrease in the cation coordination number 12 which influences the correctness of the fit that might be responsible for the observed deviation.

Conclusions
Nd-doped ceria solid solutions (Ce 1x Nd x O 2 ) with "x" ranging from 0 to 0.25 were prepared by the self-propagating room temperature reaction.The Rietveld refinement showed that the obtained powders exhibit a precise stoichiometry compared to the tailored composition.It was found that the crystallite size lies in the nanometric range (3 nm).The calculated and measured lattice parameters and average bond distances increase with higher dopant concentration.

Fig. 2 Fig. 3
Fig. 2 The structural refinement patterns of pure CeO 2 using X-ray powder diffraction data based on the cubic phase.A difference (observed -calculated) plot is shown beneath.Tick marks above the difference data indicate the reflection position.

Tab. I Fig. 4
Fig. 4 Part of X-ray diffraction patterns of Ce 1-x Nd x O 2- nanopowders.Shifting of the peaks toward lower angles with increasing dopant concentration is clearly visible.