Materials Science Forum Vol. 494 (2005) pp. 175-179 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland
Lattice Parameters of Gd-doped Ceria Electrolytes B. Matovic1, a, S. Boškovic1, b, Lj. Zivkovic2, c, M. Vlajic3, d and V. Krstic3, e 1
Vinca Institute of Nuclear Sciences, Lab. 170, Serbia and Montenegro
2
Faculty of Electronic Engineering, University of Nis, Serbia and Montenegro
3
Center for Manufacturing of Advanced Ceramics and Nanomaterials, Queens University, Nicol Hall, Kingston, Canada a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Ceria, Lattice Parameters, Nanomaterials, Solid Solution.
Abstract. This paper deals with Gd-doped ceria solid solutions: Ce1−XGdXO2−δ with "x" ranging from 0 to 0.2. Four different powders were synthesized by modified glycine nitrate procedure with very precise stoichiometry according to tailored composition. The method was modified by decreasing glycine/nitrate ratio to 0.5. All obtained solid solutions exhibit a fluorite-type crystal structure with composition dependent lattice parameters. The variation of the lattice parameter was studied and correlated with the equation describing the ion-packing model. It has been found that the change of lattice parameter versus Gd concentration obeys Vegard's rule very well. Results also show that all powders are nanometric in size. The average size of Ce1−XGdXO2−δ particles is about 20 nm. Introduction Ceria based materials exhibit excellent oxygen storage behavior. This results from the balance between reduced and oxidized states of Ce ion, i.e., Ce+3, Ce+4 and from increased oxygen transport capacity. Thus, ceria is a very promising material used 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 higher than that of stabilized zirconia [3]. Larger ionic radius of Ce4+ (0.97 Å) than of Zr4+ (0.72 Å), results in 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 Ce4+ reduction into Ce3+ or by chemical doping of ceria with other transition or rare-earth element [5]. The key factor in designing modified ceria is the choice of a dopant, as well as its amount introduced. In the case of gadolinia dopant [6], the highest values of conductivity were obtained. In addition, the preparation method of powder very strongly affects the homogeneity and stability of solid solutions. In this work the powders were prepared by a modified glycine nitrate procedure (MGNP) [7]. This technique enables production of very fine powders with very precise stoichiometry in accordance with tailored compositions. This paper describes the preparation and characterization of a number of solid solutions of gadolinia doped ceria in order to study the variation of the lattice parameter with Gd content in the low concentration region.
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Experimental Starting chemicals used for the synthesis of powders were aminoacetic acid (Glycine, Fischer Scientific, USA), and nitrates of Ce and Gd (Aldrich, USA). The glycine nitrate process was modified by changing the glycine /nitrate ratio [8] according to overall reaction: 4 C2H5NO2 + 2 [(1-x)Ce(NO3)3 6H2O + xGd(NO3)3 6H2O] + 2 O2 ⇒ ⇒ 2 Ce1-xGdxO2-y + 22 H2O(g) + 5 N2(g) + 8 CO2(g).
(1)
The following powders were synthesized: CeO2, Ce0.9Gd0.1O2-δ, Ce0.85Gd0.15O2-δ and Ce0.8Gd0.2O2-δ. X-ray analysis was used to identify the crystalline phases, as well as lattice parameter of obtained powder solid solution. The data were collected in the 20-120° 2Θ range with a step width of 0.05° and time of 10 s per step. Before measurements, the angular correction was done by high quality Si standard. In order to avoid errors due to specimen preparation, three independent determinations of the lattice parameters were performed for each composition. Lattice parameters were refined from the fitted data using the least-squares procedure [9]. Synthesized powders were also characterized by measuring the specific surface area, by chemical analysis in order to clear up the true concentration of dopants, and by SEM microscopy. Results and Discussion Ceria crystallizes into fluorite-type structure with coordination number for cations and anions, 8 and 4, respectively. An ideal fluorite structure has the lattice parameter given by: aO =
4 (rcation + ranion ) . 3
(2)
However, the doping with a definite number of lower valent cations (Gd3+) creates the corresponding number of anion vacancies due to charge compensation according to the following defect reaction: x xGd1.5 + (1 − x ) ⋅ CeO2 ⇒ xGd Ce + 0.5VO.. + (1 − x ) ⋅ CeCe + (2 − 0.5 x ) ⋅ OO.. `.
(3)
If x moles of dopant cations, Gd3+, occupy Ce4+ sites, (1-x) moles of the host cation Ce4+ will remain on their sites. This reaction also implies that O2- sites are occupied by 0.5x moles of oxygen vacancies, VO.. , and 2-0.5x moles of host anions O2-. Thus, the calculation of the lattice parameter of a solid solution with the fluorite structure of CeO2-MO1.5, where MO1.5 is the dopant, has to take into account all ionic radii (dopant cation, cerium ion, oxygen ion) including oxygen vacancy. Recently, Hong has proposed an equation for the calculation of lattice parameters of fluorite structure based on the ion-packing model [10] described by the following equation: 4 4 ao = ⋅ [rM − rCe − 0.25rO + 0.25vO ]⋅ u + ⋅ [rCe + rO ] ⋅ 0.9971. 3 3
(4)
where rM , rCe , ro and rVo are the ionic radii of the dopant cation, cerium ion, oxygen ion and oxygen vacancy, respectively; u is the dopant molar fraction. According to X-ray diffraction analysis, the obtained powders are single phased, independent of dopant concentration in the range investigated (Fig.1). Peaks related to isolated gadolinia phases are not observed, however, several weak diffraction lines of an unknown phase are
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present in all patterns. One does not expect that the presence of this minor phase has any significant influence on unit cell parameters. All solid solution powders exhibit the fluorite crystal structure. Dissolution of Gd2O3 in a cubic fluorite lattice causes a shift in ceria peaks toward lower angles indicating existence of the solid solution (Fig. 2). This coincides with the changes in lattice constant, ao (Table 1). Chemical composition and specific surface areas of the synthesized powders are given in Table 1. Determined compositions are very close to theoretically calculated ones. Also, specific surface area is very large, indicating that nano-size particles of synthesized powders were obtained. 3500
Intensity (a.u.)
3000 2500 2000 ?
?
1500
Ce0.8Gd0.2O2-δ
?
Ce0.85Gd0.15O2-δ
1000 500
Ce0.9Gd0.1O2-δ
0 20
30
40
50
60
70
80
90
100
110
120
2θ Fig. 1 X-ray diffraction patterns of Cex-1GdxO2-δ powders. ? trace of unknown phase.
222
220
2500
Intensity (a.u.)
311
3000
2000 1500 1000
Ce0.8Gd0.2O2
500
Ce0.85Gd0.15O2 Ce0.9Gd0.1O2
0 45
50
55
60
2θ Fig. 2 Part of X-ray diffraction patterns of Cex-1GdxO2-δ powders. Shifting of peaks toward lower angles with increasing dopant concentration is clearly visible. Nano-level of particle size is confirmed by SEM, too, and shown in Fig. 3. As can be seen, the particle size is about 20 nm.
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Table 1 Chemical analysis, specific surface and unit cell parameters of Ce1-xGdxO2-δ. No Composition Determined Specific surface area Unit cell designed composition [m2/g] parameters [Å] 1 CeO2 CeO2 35.63 5.4101 2 40.10 5.4129 Ce0.9Gd0.1O2-δ Ce0.9Gd0.1O2-δ 3 33.55 5.4150 Ce0.85Gd0.15O2-δ Ce0.86Gd0.14O2-δ 4 35.94 5.4179 Ce0.8Gd0.2O2-δ Ce0.792Gd0.208O2-δ
Fig. 3 SEM micrograph of Ce0.9Gd0.1O2-δ powder.
Lattice parameter, aO(Å)
Calculation of cell parameters (Fig. 4) on the basis of X-ray results, shows a linear dependence of unit cell parameter versus molar fraction, x, of Gd+3 ions. With increasing Gd ion concentration the cubic ceria lattice expands. According to Shannon's compilation [11], the ionic radii of Ce+4 and Gd+3 for CN 8, are 0.97 and 1.053 Å, respectively. Thus, doped with a larger sized Gd+3 ion and with increasing dopant concentration will keep on enlarging cell lattice. Calculated according to the 5.418
5.416
5.414
5.412
Fig. 4 Calculated (◇) and measured () lattice parameter (ao) of doped ceria as a function of Gd content in samples (Ce1-xGdxO2-δ).
5.410 0.00
0.05
0.10
X (mol)
0.15
0.20
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Eq. 4, and measured lattice parameter (ao) of doped ceria (Fig.4) versus Gd3+ content, obeys Vegard's law. The results show that there is a very good agreement between the experimental and the calculated values. A small discrepancy can be attributed to the changes in anion vacancy radius or to the decrease in cation coordination number [12]. Since in diluted solution, the radius of cation changes linearly with dopant concentration, then the anion radius must exhibit a linear dependence with increasing concentration of anion vacancies [13]. In this paper we used a fixed value of the oxygen vacancy radius (1.164 Å) [14]. It is known that fixed value for anion vacancy radius is not accurate [13] and it might be responsible for observed small deviation. Conclusions Gd-doped ceria solid solutions (Ce1−XGdXO2−δ) with "x" ranging from 0 to 0.2 prepared by modified glycine nitrate procedure show very precise stoichiometry compared to the tailored composition. It was found that the particle size lies in the nanometric range (20 nm). The calculated and measured lattice parameters were compared and found to be in good agreement. Variation of lattice parameter with increasing Gd content obeys Vegard's law and this behavior can be described with ion-packing model. Acknowledgement The authors are grateful to the Ministry of Science and Environmental Protection of the Republic of Serbia for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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