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Crystal structure, thermal expansion and electrical conductivity of dual-phase Gd0.8Sr0.2Co1−yFeyO3−δ (0≤y≤1.0)
Solid State Ionics 176 (2005) 103 – 108 www.elsevier.com/locate/ssi
Crystal structure, thermal expansion and electrical conductivity of dual-phase Gd0.8Sr0.2Co1 y Fey O3 d (0VyV1.0) C.R. Dycka, R.C. Petersonb, Z.B. Yua, V.D. Krstica,* a
Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Nicol Hall, Queen’s University, Kingston, ON, Canada K7L 3N6 b Department of Geological Sciences and Geological Engineering, Miller Hall, Queen’s University, Kingston, ON, Canada K7L 3N6 Received 14 March 2004; received in revised form 10 June 2004; accepted 18 June 2004
Abstract Compositions in the Gd0.8Sr0.2Co1 y Fey O3 d (0VyV1.0) system were synthesized and the atomic structures were refined by Rietveld analysis of X-ray powder diffraction data. Thermal expansion and electrical conductivity were determined using dilatometry and fourpoint DC, respectively. A stable dual-phase perovskite system was shown to exist for all compositions. The primary phase had an orthorhombic symmetry (Pnma, #62) and the secondary phase had a cubic symmetry (Pm-3m, #221). Sr preferentially partitioned into the secondary cubic phase. Both the thermal expansion coefficient and the electrical conductivity decreased with increasing iron content. The potential for producing stable dual-phase perovskites opens up possibilities in material design for solid oxide fuel cell (SOFC) cathodes. D 2004 Elsevier B.V. All rights reserved. Keywords: Rietveld analysis; Powder diffraction; Thermal expansion; Electrical conductivity; Perovskite
1. Introduction Perovskite oxides of the form Ln1 x Srx B1 y ByVO3 d (Ln=La, Sm, Gd, etc.; B, BV=Co, Fe, Mn) have been investigated in the past decade as potential cathode materials for intermediate temperature solid oxide fuel cells (ITSOFCs) [1–3]. In contrast to compositions where manganese occupies the perovskite B-site, the substitution of cobalt in the B-site has been demonstrated to increase the catalytic activity of the cathode for the reduction of oxygen and produces a mixed-conducting oxide [3]. A mixedconducting oxide cathode possesses a triple-phase boundary region that extends over the entire surface of the cathode, as O2 ions that are produced on the surface of the cathode may be carried to the electrolyte by virtue of cathode ionic conduction pathways. The focus to date has been largely on the empirically determined properties of IT-SOFC cathode materials. The * Corresponding author. Tel.: +1 613 533 2760; fax: +1 613 533 6610. E-mail address: [email protected] (V.D. Krstic). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.06.005
electrical conductivity and thermal expansion of the Ln1 x Srx Co1 y Fey O3 d (Ln=Pr, Nd, Gd; x=0.2, 0.3) has been reported and the values obtained in the present study for the Gd0.8Sr0.2Co1 y Fey O3 d system show good agreement with this earlier work [4]. The authors of this paper also looked at polarization of these cathode systems with a ceria-based electrolyte and found good cathodic activity with little dependence on the A-site lanthanide species. In the present work, Gd has been chosen for the A-site lanthanide because the majority of IT-SOFC electrolytes under investigation involve Gddoping of ceria. Placing the same ion in the electrolyte and the cathode serves to enhance chemical compatibility of constituent layers. Cobalt-bearing perovskites containing Gd or Pr have been described as consisting of a single-phase perovskite structure for compositions with less than 30 mol% Srdoping [5–8]. The symmetry of these perovskites has been reported as orthorhombic Pnma based on X-ray analysis. The system Gd1 x Srx CoO3 d was proposed to consist of more than one phase, but fine analysis was not carried out
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[3]. The presence of a second phase in an oxide system usually results in a drastic reduction in electrical properties, as microstructural mismatch between phases presents additional resistance as a result of increased charge carrier scattering. In this work, the Gd0.8Sr0.2Co1 y Fey O3 d system was found to form a mixture of two perovskite structures with different compositions and symmetry. The choice of 80:20 for Gd/Sr on the A-site was made after considering earlier results for this system [9]. In brief, compositions with greater than 20% Sr on the A-site suffered from poor mechanical integrity following sintering. The materials studied exhibited high electrical conductivity and were characterized by four-point DC, dilatometry, and Rietveld analysis. The potential for tailoring properties by stabilizing more reactive/unstable phases is presented. Fig. 1. X-ray powder diffraction data for Gd0.8Sr0.2Co1 y Fey O3 d powders (Co Ka radiation). Note the additional peaks ( y=0, 38.528; y=0.2, 38.428; y=0.4, 38.388) and the peak widening ( y=0.6, 38.248; y=0.8, 38.058; y=1.0, 37.798) due to the second cubic perovskite phase in the predominantly orthorhombic material.
2. Experimental Gd0.8Sr0.2Co1 y Fey O3 d powders were prepared by the glycine–nitrate process [10]. Ash powders were calcined in
Table 1 Atomic fractional coordinates (x, y, z) and occupancy of atomic sites obtained from the Rietveld refinement for the orthorhombic content of samples with overall stoichiometry, Gd0.8Sr0.2Co1 y Fey O3 d B-site Fe occupancy fraction ( y)
Atom
Wyckoff multiplicity
Occupancy fraction
x
y
z
0
Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2
4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d
0.9(1) 0.1(1) 1.0 1.0 1.0 0.9(1) 0.1(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0
0.4448(2) 0.4447(2) 0.0 0.516(2) 0.115(2) 0.4440(2) 0.4438(2) 0.0 0.514(1) 0.144(1) 0.4422(1) 0.4421(1) 0.0 0.522(1) 0.172(1) 0.4417(1) 0.4416(1) 0.0 0.531(1) 0.205(1) 0.4421(1) 0.4420(1) 0.0 0.541(2) 0.225(2) 0.4420(1) 0.4419(1) 0.0 0.547(2) 0.225(2)
0.25 0.25 0.0 0.25 0.045(1) 0.25 0.25 0.0 0.25 0.039(1) 0.25 0.25 0.0 0.25 0.034(1) 0.25 0.25 0.0 0.25 0.0453(9) 0.25 0.25 0.0 0.25 0.051(1) 0.25 0.25 0.0 0.25 0.052(1)
0.0138(5) 0.0139(5) 0.0 0.578(2) 0.276(2) 0.0138(4) 0.0138(4) 0.0 0.580(2) 0.287(1) 0.0151(3) 0.0151(3) 0.0 0.601(2) 0.292(1) 0.0148(3) 0.0149(3) 0.0 0.621(1) 0.277(2) 0.9866(3) 0.9868(3) 0.0 0.368(2) 0.273(2) 0.9839(3) 0.9841(3) 0.0 0.373(2) 0.275(2)
0.2
0.4
0.6
0.8
1
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
C.R. Dyck et al. / Solid State Ionics 176 (2005) 103–108
air at 900 8C followed by ball milling for 20 h in ethanol using zirconia media. Polyethylene glycol binder was added in the amount of 5 wt.% to each powder in the final hour of milling. Powders were dried and sieved to produce the final reaction powders. For thermal expansion and conductivity tests, dense samples were prepared by uniaxially pressing powders under 20 MPa pressure followed by cold isostatic pressing to 200 MPa to form rectangular green compacts with nominal dimensions 1544 mm. Dense Gd0.8Sr0.2Co1 y Fey O3 d samples were obtained by sintering the compacts at 1200 8C for 4 h in air (heating/cooling rate: 2 8C/min above 900 8C). Powders for X-ray analysis were prepared by heating 2 g of ash powder to 1200 8C for 4 h in air. X-ray patterns from backpacked samples were collected from 208 to 1108 2h using a high-resolution Philips X’Pertn diffractometer with CoKa radiation and an X’celeratorn position sensitive detector. Atomic structure analysis using the Rietveld method was performed using the X’Pert Plusn software package. Initial modelling as a single-phase perovskite revealed the necessity for adding a second phase to the calculations. Thermal expansion measurements were performed at a heating rate of 2 8C/min over the range 50–1000 8C using a Netzch dilatometer calibrated to a quartz standard. Electrical conductivity measurements were made using a four-point DC conductivity apparatus calibrated with copper (room temperature) and silver (25–700 8C). A steady value of the DC conductivity was reached prior to the collection of each measurement. Platinum wire leads were attached to the samples and good connections were ensured by applying silver paste (Ferro). Bulk conductivity was measured over the temperature range 25–700 8C.
achieved for all compositions. The atomic fractional coordinates determined by least-squares refinement of powder diffraction data for the orthorhombic and cubic perovskite phases are shown in Tables 1 and 2, respectively. The atomic ratio of Gd to Sr was obtained by refinement of the site occupancy by assuming that only Gd and Sr occupy the A-site and that the site was fully occupied as the Fe and Co content of the B-site was varied. It is not possible to perform a similar site refinement to determine the Co to Fe ratio on the B-site because Co and Fe have very similar electronic structure and the X-ray experiment is not sensitive to this difference. It is not expected that any significant ordering of the B-site would occur as a result of the varying ratio of Co to Fe due to their similar size and charge and the model assumes that the B-site content of both the orthorhombic and cubic perovskite structures is that of the starting materials. A detailed TEM/electron diffraction study, beyond the scope of the current study, would be the next step to sort out the contribution of each of the ions on the B-site. The relative amount of the two phases present may be obtained from the ratio of the refined scale factor and knowing the unit cell volume, atomic weight of a formula unit and the number of formula units within the unit cell. It is clear from Tables 1 and 2 that Sr is preferentially partitioned
Table 2 Atomic fractional coordinates (x, y, z) and occupancy of atomic sites obtained from the Rietveld refinement for the cubic content of samples with overall stoichiometry, Gd0.8Sr0.2Fe1 y Coy O3 d B-site Fe occupancy fraction ( y)
Atom
Wyckoff multiplicity
Occupancy fraction
x
y
z
0
Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O
1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d
0.51(3) 0.49(3) 1.0 1.0 0.55(3) 0.45(3) 1.0 1.0 0.33(4) 0.67(4) 1.0 1.0 0.42(6) 0.58(6) 1.0 1.0 0.50(9) 0.50(9) 1.0 1.0 0.4(1) 0.6(1) 1.0 1.0
0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0
0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0
0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5
3. Results and discussion 0.2
3.1. Rietveld analysis The presence of more than one phase was detected for all compositions in the Gd0.8Sr0.2Co1 y Fey O3 d system. The X-ray diffraction data for six different compositions is shown in Fig. 1. It can be seen clearly that for y=0, an additional peak is present at 38.58. This peak is evident as a separate entity until y=0.4, above which its contribution is to increase the overall area of the peaks situated between 37.88 and 38.28. It was initially thought that this additional peak may be the result of a lowering of the symmetry to a subgroup of Pnma such as monoclinic P21/c. However, it was not possible to index all lines in the pattern using a single unit cell. A model in which the additional peaks were attributed to a second phase of cubic perovskite coexisting with the orthorhombic phase in a stable configuration was introduced into the Rietveld calculations. This model was successful and convergence was
105
0.4
0.6
0.8
1
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
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Table 3 Internal consistency assessment of the Rietveld refinement results for comparison with the ratio of Gd/Sr on the A-site in the starting material (0.8:0.2) in Gd0.8Sr0.2Fe1 y Coy O3 d B-site Fe occupancy fraction ( y)
Sr mole fraction
Orthorhombic Mole fraction of total sample
Cubic Sr mole fraction
Mole fraction of total sample
0.00 0.20 0.40 0.60 0.80 1.00 Averages
0.10 0.10 0.00 0.00 0.00 0.00 0.03
0.59 0.57 0.53 0.65 0.72 0.63
0.49 0.45 0.67 0.59 0.51 0.60 0.55
0.41 0.43 0.47 0.35 0.28 0.37
into the cubic phase. The two phases had nominal compositions of Gd0.45Sr0.55Co1 y Fey O3 d and GdCo1 y Fey O3 d for cubic and orthorhombic phases, respectively (Table 3). When the compositions determined by Rietveld analysis are combined with the relative abundances determined by Rietveld analysis, the bulk composition of the sample closely matches the starting composition. The starting material had a Gd/Sr ratio target of 0.80:0.20 mole fraction and the resulting internal consistency assessment of the refinement gave a ratio 0.77:0.23, as presented in Table 3. It was found in a previous study that high Sr additions (N20 mol% on the perovskite A-site) resulted in a loss of mechanical stability of the sintered specimens [9]. In contrast, the presence of the high Sr content cubic phase within a matrix of low Sr content orthorhombic phase gave mechanically stable samples. Therefore, the dual perovskite phase structure formed stabilized the high Sr content cubic
Refined total Gd
Refined total Sr
0.74 0.75 0.69 0.79 0.86 0.78 0.77
0.26 0.25 0.31 0.21 0.14 0.22 0.23
phase while maintaining the high mechanical stability of the orthorhombic matrix phase. 3.2. Thermal expansion The addition of Fe to the perovskite B-site served to decrease the thermal expansion coefficient from that observed for the cobalt end-member (Gd0.8Sr0.2CoO3 d ). The results of the dilatometry study are presented in Fig. 2 and are reported as percent elongation vs. temperature. The thermal expansion coefficients (the slope of the lines in Fig. 2) decrease steadily with increasing Fe addition until the maximum reduction is reached at y=0.8. Completely replacing Co by Fe results in a slight increase in the thermal expansion coefficient on going from y=0.8 to 1.0. The thermal expansion coefficients for y=0.8 and 1.0 are most closely matched to the common IT-SOFC electrolyte material Ce0.8Gd0.2O1.95.
Fig. 2. Percent elongation as a function of temperature for Gd0.8Sr0.2Co1 y Fey O3
d.
C.R. Dyck et al. / Solid State Ionics 176 (2005) 103–108
107
Fig. 3. Average orthorhombic (Co/Fe)–O bond lengths, cubic (Co/Fe)–O bond lengths and thermal expansion coefficient (C.T.E.) as a function of composition for Gd0.8Sr0.2Co1 y Fey O3 d .
The thermal expansion coefficient was shown to vary as the inverse of the bond lengths between the B-site transition metal ions and oxygen. This is consistent with the ionic radii of Co2+=0.735 2 and Fe2+=0.77 2 [11]. Both the cubic and orthorhombic (average) bond lengths and the thermal expansion coefficient are shown in Fig. 3 as a function of composition. As the bond lengths directly determine the unit cell dimensions, this result is consistent with the findings of Ruffa in which the thermal expansion coefficient was determined to vary inversely with the unit cell volume [12]. As further verification, the lattice parameters are quoted for each composition in Table 4 as well as their product, the unit cell volume. 3.3. Electrical conductivity In a previous study, compositions with Sr content above 30 mol% on the perovskite A-site have been shown to exhibit high electrical conductivity on the order of 500 S
cm 1 at 600 8C in the Gd1 x Srx CoO3 d system, whereas samples with no Sr additions in the same system show electrical conductivity below 300 S cm 1 [9]. High Sr content compositions assume the cubic symmetry whereas samples without Sr assume the orthorhombic symmetry. It is significant that in this study it was shown that a mixture of the two symmetries leads to property averaging as opposed to complete degradation of electrical conductivity. For Gd0.8Sr0.2CoO3 d , the electrical conductivity at 600 8C was approximately 404 S cm 1, even though the cubic phase with higher electrical conductivity was the minority phase and comprised less than 40 wt.% for most compositions. The result is that the properties of the cubic phase with higher conduction are likely dominant when a dualphase Gd 0.8 Sr 0.2 Co 1 y Fe y O 3 d perovskite system is formed. Within the Gd0.8Sr0.2Co1 y Fey O3 d system, the electrical conductivity dropped as Fe was added in increasing amounts to a level of approximately 10 S cm 1 for y=0.8
Table 4 Lattice parameters and unit cell volumes for orthorhombic and cubic phases as a function of composition in Gd0.8Sr0.2Fe1 y Coy O3
d
B-site Fe occupancy fraction ( y)
Orthorhombic phase
Cubic phase
a, 2
b, 2
c, 2
Unit cell volume, 23
a=b=c, 2
Unit cell volume, 23
0 0.2 0.4 0.6 0.8 1
5.4013(2) 5.43971(8) 5.47978(9) 5.5178(1) 5.5505(1) 5.5804(1)
7.4519(2) 7.4982(1) 7.5474(1) 7.5959(1) 7.6412(2) 7.6817(2)
5.2268(1) 5.25825(7) 5.28878(9) 5.3166(1) 5.3457(1) 5.3670(1)
210.4 214.5 218.7 222.9 226.8 230.1
3.78649(8) 3.79800(4) 3.80941(5) 3.82263(8) 3.8388(1) 3.8612(2)
54.3 54.8 55.3 55.9 56.6 57.6
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
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C.R. Dyck et al. / Solid State Ionics 176 (2005) 103–108
Fig. 4. Electrical conductivity as a function of operating temperature for Gd0.8Sr0.2Co1 y Fey O3
d.
and 1.0, as shown in Fig. 4. Fe additions therefore have a deleterious effect on the electrical conductivity of LnCoO3based perovskite oxides. Fe is commonly used in lanthanum-based LnCoO3 perovskites to modify the thermal expansion coefficient; however, this comes at a great expense in terms of reduced electrical conductivity [13].
Acknowledgements
4. Conclusions
References
All compositions in the Gd0.8Sr0.2Co1 y Fey O3 d system were shown to consist of two perovskite phases; a Gddominant orthorhombic phase and a Sr-rich cubic phase. The high Sr content cubic phase was stabilized by the low Sr content orthorhombic phase and the resulting electrical conductivity was a weighted average of the properties observed in the pure cubic and orthorhombic phases. The lattice parameters of both perovskite phases were shown to expand with increasing iron content. Adding iron to the Bsite of the perovskite Gd0.8Sr0.2CoO3 d had two main effects. First, the thermal expansion decreased to levels that are more closely matched to the proposed IT-SOFC electrolyte Ce0.8Gd0.2O2 d for the high Fe-content compositions of Gd0.8Sr0.2Co0.2Fe0.8O3 d and Gd0.8Sr0.2FeO3 d . Second, the additions of Fe caused the electrical conductivity to decrease to levels on the order of 10 S cm 1 for Gd 0.8Sr0.2Co0.2Fe0.8O3 d from 404 S cm 1 for Gd0.8Sr0.2CoO3 d .
[1] Y. Ohno, S. Nagata, H. Sato, Solid State Ionics 9/10 (1983) 1001. [2] C. Xia, W. Rauch, F. Chen, M. Liu, Solid State Ionics 149 (2002) 11. [3] Y. Takeda, H. Ueno, N. Imanishi, O. Yamamoto, N. Sammes, M.B. Phillips, Solid State Ionics 86–88 (1996) 1187. [4] L. Qiu, T. Ichikawa, A. Hirano, N. Imanishi, Y. Takeda, Solid State Ionics 158 (2003) 55 – 65. [5] H.W. Brinks, H. Fjellvag, A. Kjekshus, B.C. Hauback, J. Solid State Chem. 147 (1999) 464. [6] K.H. Ryu, K.S. Roh, S.J. Lee, C.H. Yo, J. Solid State Chem. 105 (1993) 550. [7] G.Ch. Kostogloudis, Ch. Ftikos, Solid State Ionics 135 (2000) 537. [8] M.B. Phillipps, N.M. Sammes, O. Yamamoto, Solid State Ionics 123 (1999) 131. [9] C.R. Dyck, Z.B. Yu, V.D. Krstic, Solid State Ionics 171 (2004) 1. [10] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [11] R.D. Shannon, C.T. Prewitt, Acta Crystallogr., B 25 (1969) 925. [12] A.R. Ruffa, J. Mater. Sci 15 (1980) 2258. [13] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719.
The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Strategic Skills Investment (SSI) program.
Crystal structure, thermal expansion and electrical conductivity of dual-phase Gd0.8Sr0.2Co1 y Fey O3 d (0VyV1.0) C.R. Dycka, R.C. Petersonb, Z.B. Yua, V.D. Krstica,* a
Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Nicol Hall, Queen’s University, Kingston, ON, Canada K7L 3N6 b Department of Geological Sciences and Geological Engineering, Miller Hall, Queen’s University, Kingston, ON, Canada K7L 3N6 Received 14 March 2004; received in revised form 10 June 2004; accepted 18 June 2004
Abstract Compositions in the Gd0.8Sr0.2Co1 y Fey O3 d (0VyV1.0) system were synthesized and the atomic structures were refined by Rietveld analysis of X-ray powder diffraction data. Thermal expansion and electrical conductivity were determined using dilatometry and fourpoint DC, respectively. A stable dual-phase perovskite system was shown to exist for all compositions. The primary phase had an orthorhombic symmetry (Pnma, #62) and the secondary phase had a cubic symmetry (Pm-3m, #221). Sr preferentially partitioned into the secondary cubic phase. Both the thermal expansion coefficient and the electrical conductivity decreased with increasing iron content. The potential for producing stable dual-phase perovskites opens up possibilities in material design for solid oxide fuel cell (SOFC) cathodes. D 2004 Elsevier B.V. All rights reserved. Keywords: Rietveld analysis; Powder diffraction; Thermal expansion; Electrical conductivity; Perovskite
1. Introduction Perovskite oxides of the form Ln1 x Srx B1 y ByVO3 d (Ln=La, Sm, Gd, etc.; B, BV=Co, Fe, Mn) have been investigated in the past decade as potential cathode materials for intermediate temperature solid oxide fuel cells (ITSOFCs) [1–3]. In contrast to compositions where manganese occupies the perovskite B-site, the substitution of cobalt in the B-site has been demonstrated to increase the catalytic activity of the cathode for the reduction of oxygen and produces a mixed-conducting oxide [3]. A mixedconducting oxide cathode possesses a triple-phase boundary region that extends over the entire surface of the cathode, as O2 ions that are produced on the surface of the cathode may be carried to the electrolyte by virtue of cathode ionic conduction pathways. The focus to date has been largely on the empirically determined properties of IT-SOFC cathode materials. The * Corresponding author. Tel.: +1 613 533 2760; fax: +1 613 533 6610. E-mail address: [email protected] (V.D. Krstic). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.06.005
electrical conductivity and thermal expansion of the Ln1 x Srx Co1 y Fey O3 d (Ln=Pr, Nd, Gd; x=0.2, 0.3) has been reported and the values obtained in the present study for the Gd0.8Sr0.2Co1 y Fey O3 d system show good agreement with this earlier work [4]. The authors of this paper also looked at polarization of these cathode systems with a ceria-based electrolyte and found good cathodic activity with little dependence on the A-site lanthanide species. In the present work, Gd has been chosen for the A-site lanthanide because the majority of IT-SOFC electrolytes under investigation involve Gddoping of ceria. Placing the same ion in the electrolyte and the cathode serves to enhance chemical compatibility of constituent layers. Cobalt-bearing perovskites containing Gd or Pr have been described as consisting of a single-phase perovskite structure for compositions with less than 30 mol% Srdoping [5–8]. The symmetry of these perovskites has been reported as orthorhombic Pnma based on X-ray analysis. The system Gd1 x Srx CoO3 d was proposed to consist of more than one phase, but fine analysis was not carried out
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[3]. The presence of a second phase in an oxide system usually results in a drastic reduction in electrical properties, as microstructural mismatch between phases presents additional resistance as a result of increased charge carrier scattering. In this work, the Gd0.8Sr0.2Co1 y Fey O3 d system was found to form a mixture of two perovskite structures with different compositions and symmetry. The choice of 80:20 for Gd/Sr on the A-site was made after considering earlier results for this system [9]. In brief, compositions with greater than 20% Sr on the A-site suffered from poor mechanical integrity following sintering. The materials studied exhibited high electrical conductivity and were characterized by four-point DC, dilatometry, and Rietveld analysis. The potential for tailoring properties by stabilizing more reactive/unstable phases is presented. Fig. 1. X-ray powder diffraction data for Gd0.8Sr0.2Co1 y Fey O3 d powders (Co Ka radiation). Note the additional peaks ( y=0, 38.528; y=0.2, 38.428; y=0.4, 38.388) and the peak widening ( y=0.6, 38.248; y=0.8, 38.058; y=1.0, 37.798) due to the second cubic perovskite phase in the predominantly orthorhombic material.
2. Experimental Gd0.8Sr0.2Co1 y Fey O3 d powders were prepared by the glycine–nitrate process [10]. Ash powders were calcined in
Table 1 Atomic fractional coordinates (x, y, z) and occupancy of atomic sites obtained from the Rietveld refinement for the orthorhombic content of samples with overall stoichiometry, Gd0.8Sr0.2Co1 y Fey O3 d B-site Fe occupancy fraction ( y)
Atom
Wyckoff multiplicity
Occupancy fraction
x
y
z
0
Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2 Gd Sr Co O1 O2
4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d 4c 4c 4a 4c 8d
0.9(1) 0.1(1) 1.0 1.0 1.0 0.9(1) 0.1(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0 1.0(1) 0.0(1) 1.0 1.0 1.0
0.4448(2) 0.4447(2) 0.0 0.516(2) 0.115(2) 0.4440(2) 0.4438(2) 0.0 0.514(1) 0.144(1) 0.4422(1) 0.4421(1) 0.0 0.522(1) 0.172(1) 0.4417(1) 0.4416(1) 0.0 0.531(1) 0.205(1) 0.4421(1) 0.4420(1) 0.0 0.541(2) 0.225(2) 0.4420(1) 0.4419(1) 0.0 0.547(2) 0.225(2)
0.25 0.25 0.0 0.25 0.045(1) 0.25 0.25 0.0 0.25 0.039(1) 0.25 0.25 0.0 0.25 0.034(1) 0.25 0.25 0.0 0.25 0.0453(9) 0.25 0.25 0.0 0.25 0.051(1) 0.25 0.25 0.0 0.25 0.052(1)
0.0138(5) 0.0139(5) 0.0 0.578(2) 0.276(2) 0.0138(4) 0.0138(4) 0.0 0.580(2) 0.287(1) 0.0151(3) 0.0151(3) 0.0 0.601(2) 0.292(1) 0.0148(3) 0.0149(3) 0.0 0.621(1) 0.277(2) 0.9866(3) 0.9868(3) 0.0 0.368(2) 0.273(2) 0.9839(3) 0.9841(3) 0.0 0.373(2) 0.275(2)
0.2
0.4
0.6
0.8
1
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
C.R. Dyck et al. / Solid State Ionics 176 (2005) 103–108
air at 900 8C followed by ball milling for 20 h in ethanol using zirconia media. Polyethylene glycol binder was added in the amount of 5 wt.% to each powder in the final hour of milling. Powders were dried and sieved to produce the final reaction powders. For thermal expansion and conductivity tests, dense samples were prepared by uniaxially pressing powders under 20 MPa pressure followed by cold isostatic pressing to 200 MPa to form rectangular green compacts with nominal dimensions 1544 mm. Dense Gd0.8Sr0.2Co1 y Fey O3 d samples were obtained by sintering the compacts at 1200 8C for 4 h in air (heating/cooling rate: 2 8C/min above 900 8C). Powders for X-ray analysis were prepared by heating 2 g of ash powder to 1200 8C for 4 h in air. X-ray patterns from backpacked samples were collected from 208 to 1108 2h using a high-resolution Philips X’Pertn diffractometer with CoKa radiation and an X’celeratorn position sensitive detector. Atomic structure analysis using the Rietveld method was performed using the X’Pert Plusn software package. Initial modelling as a single-phase perovskite revealed the necessity for adding a second phase to the calculations. Thermal expansion measurements were performed at a heating rate of 2 8C/min over the range 50–1000 8C using a Netzch dilatometer calibrated to a quartz standard. Electrical conductivity measurements were made using a four-point DC conductivity apparatus calibrated with copper (room temperature) and silver (25–700 8C). A steady value of the DC conductivity was reached prior to the collection of each measurement. Platinum wire leads were attached to the samples and good connections were ensured by applying silver paste (Ferro). Bulk conductivity was measured over the temperature range 25–700 8C.
achieved for all compositions. The atomic fractional coordinates determined by least-squares refinement of powder diffraction data for the orthorhombic and cubic perovskite phases are shown in Tables 1 and 2, respectively. The atomic ratio of Gd to Sr was obtained by refinement of the site occupancy by assuming that only Gd and Sr occupy the A-site and that the site was fully occupied as the Fe and Co content of the B-site was varied. It is not possible to perform a similar site refinement to determine the Co to Fe ratio on the B-site because Co and Fe have very similar electronic structure and the X-ray experiment is not sensitive to this difference. It is not expected that any significant ordering of the B-site would occur as a result of the varying ratio of Co to Fe due to their similar size and charge and the model assumes that the B-site content of both the orthorhombic and cubic perovskite structures is that of the starting materials. A detailed TEM/electron diffraction study, beyond the scope of the current study, would be the next step to sort out the contribution of each of the ions on the B-site. The relative amount of the two phases present may be obtained from the ratio of the refined scale factor and knowing the unit cell volume, atomic weight of a formula unit and the number of formula units within the unit cell. It is clear from Tables 1 and 2 that Sr is preferentially partitioned
Table 2 Atomic fractional coordinates (x, y, z) and occupancy of atomic sites obtained from the Rietveld refinement for the cubic content of samples with overall stoichiometry, Gd0.8Sr0.2Fe1 y Coy O3 d B-site Fe occupancy fraction ( y)
Atom
Wyckoff multiplicity
Occupancy fraction
x
y
z
0
Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O Gd Sr Fe/Co O
1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d 1b 1a 1a 3d
0.51(3) 0.49(3) 1.0 1.0 0.55(3) 0.45(3) 1.0 1.0 0.33(4) 0.67(4) 1.0 1.0 0.42(6) 0.58(6) 1.0 1.0 0.50(9) 0.50(9) 1.0 1.0 0.4(1) 0.6(1) 1.0 1.0
0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0
0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0 0.5 0.5 0 0
0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5 0.5 0.5 0 0.5
3. Results and discussion 0.2
3.1. Rietveld analysis The presence of more than one phase was detected for all compositions in the Gd0.8Sr0.2Co1 y Fey O3 d system. The X-ray diffraction data for six different compositions is shown in Fig. 1. It can be seen clearly that for y=0, an additional peak is present at 38.58. This peak is evident as a separate entity until y=0.4, above which its contribution is to increase the overall area of the peaks situated between 37.88 and 38.28. It was initially thought that this additional peak may be the result of a lowering of the symmetry to a subgroup of Pnma such as monoclinic P21/c. However, it was not possible to index all lines in the pattern using a single unit cell. A model in which the additional peaks were attributed to a second phase of cubic perovskite coexisting with the orthorhombic phase in a stable configuration was introduced into the Rietveld calculations. This model was successful and convergence was
105
0.4
0.6
0.8
1
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
106
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Table 3 Internal consistency assessment of the Rietveld refinement results for comparison with the ratio of Gd/Sr on the A-site in the starting material (0.8:0.2) in Gd0.8Sr0.2Fe1 y Coy O3 d B-site Fe occupancy fraction ( y)
Sr mole fraction
Orthorhombic Mole fraction of total sample
Cubic Sr mole fraction
Mole fraction of total sample
0.00 0.20 0.40 0.60 0.80 1.00 Averages
0.10 0.10 0.00 0.00 0.00 0.00 0.03
0.59 0.57 0.53 0.65 0.72 0.63
0.49 0.45 0.67 0.59 0.51 0.60 0.55
0.41 0.43 0.47 0.35 0.28 0.37
into the cubic phase. The two phases had nominal compositions of Gd0.45Sr0.55Co1 y Fey O3 d and GdCo1 y Fey O3 d for cubic and orthorhombic phases, respectively (Table 3). When the compositions determined by Rietveld analysis are combined with the relative abundances determined by Rietveld analysis, the bulk composition of the sample closely matches the starting composition. The starting material had a Gd/Sr ratio target of 0.80:0.20 mole fraction and the resulting internal consistency assessment of the refinement gave a ratio 0.77:0.23, as presented in Table 3. It was found in a previous study that high Sr additions (N20 mol% on the perovskite A-site) resulted in a loss of mechanical stability of the sintered specimens [9]. In contrast, the presence of the high Sr content cubic phase within a matrix of low Sr content orthorhombic phase gave mechanically stable samples. Therefore, the dual perovskite phase structure formed stabilized the high Sr content cubic
Refined total Gd
Refined total Sr
0.74 0.75 0.69 0.79 0.86 0.78 0.77
0.26 0.25 0.31 0.21 0.14 0.22 0.23
phase while maintaining the high mechanical stability of the orthorhombic matrix phase. 3.2. Thermal expansion The addition of Fe to the perovskite B-site served to decrease the thermal expansion coefficient from that observed for the cobalt end-member (Gd0.8Sr0.2CoO3 d ). The results of the dilatometry study are presented in Fig. 2 and are reported as percent elongation vs. temperature. The thermal expansion coefficients (the slope of the lines in Fig. 2) decrease steadily with increasing Fe addition until the maximum reduction is reached at y=0.8. Completely replacing Co by Fe results in a slight increase in the thermal expansion coefficient on going from y=0.8 to 1.0. The thermal expansion coefficients for y=0.8 and 1.0 are most closely matched to the common IT-SOFC electrolyte material Ce0.8Gd0.2O1.95.
Fig. 2. Percent elongation as a function of temperature for Gd0.8Sr0.2Co1 y Fey O3
d.
C.R. Dyck et al. / Solid State Ionics 176 (2005) 103–108
107
Fig. 3. Average orthorhombic (Co/Fe)–O bond lengths, cubic (Co/Fe)–O bond lengths and thermal expansion coefficient (C.T.E.) as a function of composition for Gd0.8Sr0.2Co1 y Fey O3 d .
The thermal expansion coefficient was shown to vary as the inverse of the bond lengths between the B-site transition metal ions and oxygen. This is consistent with the ionic radii of Co2+=0.735 2 and Fe2+=0.77 2 [11]. Both the cubic and orthorhombic (average) bond lengths and the thermal expansion coefficient are shown in Fig. 3 as a function of composition. As the bond lengths directly determine the unit cell dimensions, this result is consistent with the findings of Ruffa in which the thermal expansion coefficient was determined to vary inversely with the unit cell volume [12]. As further verification, the lattice parameters are quoted for each composition in Table 4 as well as their product, the unit cell volume. 3.3. Electrical conductivity In a previous study, compositions with Sr content above 30 mol% on the perovskite A-site have been shown to exhibit high electrical conductivity on the order of 500 S
cm 1 at 600 8C in the Gd1 x Srx CoO3 d system, whereas samples with no Sr additions in the same system show electrical conductivity below 300 S cm 1 [9]. High Sr content compositions assume the cubic symmetry whereas samples without Sr assume the orthorhombic symmetry. It is significant that in this study it was shown that a mixture of the two symmetries leads to property averaging as opposed to complete degradation of electrical conductivity. For Gd0.8Sr0.2CoO3 d , the electrical conductivity at 600 8C was approximately 404 S cm 1, even though the cubic phase with higher electrical conductivity was the minority phase and comprised less than 40 wt.% for most compositions. The result is that the properties of the cubic phase with higher conduction are likely dominant when a dualphase Gd 0.8 Sr 0.2 Co 1 y Fe y O 3 d perovskite system is formed. Within the Gd0.8Sr0.2Co1 y Fey O3 d system, the electrical conductivity dropped as Fe was added in increasing amounts to a level of approximately 10 S cm 1 for y=0.8
Table 4 Lattice parameters and unit cell volumes for orthorhombic and cubic phases as a function of composition in Gd0.8Sr0.2Fe1 y Coy O3
d
B-site Fe occupancy fraction ( y)
Orthorhombic phase
Cubic phase
a, 2
b, 2
c, 2
Unit cell volume, 23
a=b=c, 2
Unit cell volume, 23
0 0.2 0.4 0.6 0.8 1
5.4013(2) 5.43971(8) 5.47978(9) 5.5178(1) 5.5505(1) 5.5804(1)
7.4519(2) 7.4982(1) 7.5474(1) 7.5959(1) 7.6412(2) 7.6817(2)
5.2268(1) 5.25825(7) 5.28878(9) 5.3166(1) 5.3457(1) 5.3670(1)
210.4 214.5 218.7 222.9 226.8 230.1
3.78649(8) 3.79800(4) 3.80941(5) 3.82263(8) 3.8388(1) 3.8612(2)
54.3 54.8 55.3 55.9 56.6 57.6
Errors are reported in brackets as one standard deviation and have the same order of magnitude as the smallest decimal place.
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Fig. 4. Electrical conductivity as a function of operating temperature for Gd0.8Sr0.2Co1 y Fey O3
d.
and 1.0, as shown in Fig. 4. Fe additions therefore have a deleterious effect on the electrical conductivity of LnCoO3based perovskite oxides. Fe is commonly used in lanthanum-based LnCoO3 perovskites to modify the thermal expansion coefficient; however, this comes at a great expense in terms of reduced electrical conductivity [13].
Acknowledgements
4. Conclusions
References
All compositions in the Gd0.8Sr0.2Co1 y Fey O3 d system were shown to consist of two perovskite phases; a Gddominant orthorhombic phase and a Sr-rich cubic phase. The high Sr content cubic phase was stabilized by the low Sr content orthorhombic phase and the resulting electrical conductivity was a weighted average of the properties observed in the pure cubic and orthorhombic phases. The lattice parameters of both perovskite phases were shown to expand with increasing iron content. Adding iron to the Bsite of the perovskite Gd0.8Sr0.2CoO3 d had two main effects. First, the thermal expansion decreased to levels that are more closely matched to the proposed IT-SOFC electrolyte Ce0.8Gd0.2O2 d for the high Fe-content compositions of Gd0.8Sr0.2Co0.2Fe0.8O3 d and Gd0.8Sr0.2FeO3 d . Second, the additions of Fe caused the electrical conductivity to decrease to levels on the order of 10 S cm 1 for Gd 0.8Sr0.2Co0.2Fe0.8O3 d from 404 S cm 1 for Gd0.8Sr0.2CoO3 d .
[1] Y. Ohno, S. Nagata, H. Sato, Solid State Ionics 9/10 (1983) 1001. [2] C. Xia, W. Rauch, F. Chen, M. Liu, Solid State Ionics 149 (2002) 11. [3] Y. Takeda, H. Ueno, N. Imanishi, O. Yamamoto, N. Sammes, M.B. Phillips, Solid State Ionics 86–88 (1996) 1187. [4] L. Qiu, T. Ichikawa, A. Hirano, N. Imanishi, Y. Takeda, Solid State Ionics 158 (2003) 55 – 65. [5] H.W. Brinks, H. Fjellvag, A. Kjekshus, B.C. Hauback, J. Solid State Chem. 147 (1999) 464. [6] K.H. Ryu, K.S. Roh, S.J. Lee, C.H. Yo, J. Solid State Chem. 105 (1993) 550. [7] G.Ch. Kostogloudis, Ch. Ftikos, Solid State Ionics 135 (2000) 537. [8] M.B. Phillipps, N.M. Sammes, O. Yamamoto, Solid State Ionics 123 (1999) 131. [9] C.R. Dyck, Z.B. Yu, V.D. Krstic, Solid State Ionics 171 (2004) 1. [10] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [11] R.D. Shannon, C.T. Prewitt, Acta Crystallogr., B 25 (1969) 925. [12] A.R. Ruffa, J. Mater. Sci 15 (1980) 2258. [13] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719.
The authors would like to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Strategic Skills Investment (SSI) program.