Solid State Ionics 192 (2011) 245–247

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Solid State Ionics

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Oxygen relaxation and phase transition in GdBaCo2O5+ δ oxide

Wei Liu a,⁎, Chunli Yang a, Xiusheng Wu b, Haiying Gao a, Zhijun Chen a

a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR Chinab School of Materials & Chemical Engineering, Anhui University of Architecture, Hefei 230022, PR China

⁎ Corresponding author. Tel.: +86 551 3606929; fax:E-mail address: wliu@ustc.edu.cn (W. Liu).

0167-2738/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.ssi.2010.04.028

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 August 2009Received in revised form 25 February 2010Accepted 26 April 2010Available online 20 May 2010

Keywords:Electrical conductivityOxygen relaxationPhase transitionInternal friction

Electrical conductivity, internal friction techniques and dilatometer have been used to investigate the oxygenrelaxation, phase transition and thermal expansion behavior of GdBaCo2O5+ δ. The main electronic chargecarriers in GdBaCo2O5+δ are electronic holes, which could be assigned to the formation of Co4+. The oxygenexchange kinetics intensely depends on oxygen partial pressure and is also closely related to temperature.Both electrical conductivity and internal friction give rise to an abnormal at about 75 °C, which are related tothe insulator-metal transition occurring in GdBaCo2O5+ δ. One large relaxation internal friction peak, due tothe motion of oxygen within Gd–O plane, is also found in the oxide. The average thermal expansioncoefficient (TEC) of GdBaCo2O5+ δ is about 21.4×10−6 K−1 between 500 °C and 900 °C.

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1. Introduction

Perovskite oxides are a very important class of materials due totheir application as catalysts, magnetic and electric materials. Amongthem, Cobalt oxides such as La1-xSrxCo1-yFeyO3-δ have been widelyinvestigated as oxygen-permeable dense ceramic membranes andcathode materials for solid oxide fuel (SOFC). More recently, a greatinterest about the layered RBaCo2O5+ δ (R=rare earth elements;0≤δ≤1) cobaltites has appeared. They present several spectacularproperties such as spin-state, metal-insulator, and magnetic-field-induced transitions as well as charge and orbital ordering phenomena[1–6]. The crystal structure of these oxides is formed by stackingsequence [ROδ]–[CoO2]–[BaO]–[CoO2] along the c direction. Transfor-mation of a simple cubic perovskite, with randomly occupied A sites,into a layered crystal with alternating lanthanide and alkali-earthplanes reduces the oxygen bonding strength and provides disorder-free channels for ion motion, pointing to an efficient way to designnew ionic conductors [5]. The mobile oxygen, which is located withinthe Gd–O plane, can be reversibly added and removed from thesystem in the vicinity of 600 K [6], providing a variability of theoxygen content.

Thermogravimetric and electrical conductivity relaxation analysishave revealed that RBaCo2O5+ δ have unusually rapid oxygentransport kinetics and high electronic conductivities at relativelylow temperature [4–7]. Recently, layered GdBaCo2O5+ δ have beenused as cathode materials for intermediate temperature solid state

oxide fuel cells (IT-SOFCs) [8–11]. In addition, GdBaCo2O5+ δ

membranes show appreciable oxygen permeability at moderatetemperatures [12,13]. In order to employ this composition either asoxide electrodes or as oxygen-permeating membranes, it is ofimportance to examine the conductivity and thermal expansionbehavior. In this paper, the oxygen relaxation, phase transition andthermal expansion behavior of GdBaCo2O5+ δ were investigated.

2. Experimental

GdBaCo2O5+ δ powers were prepared by a standard solid statereaction method. Appropriate amounts of reagent-grade Gd2O3,BaCO3, and Co2O3 powders were thoroughly mixed by ball-millingfor 20 h in ethanol using ZrO2 balls and calcined in air at 950 °C,1000 °C and 1050 °C for 10 h with intermediate grinding. The calcinedpowder was uniaxially pressed into rectangular-shape compacts,subsequently cold isostatically pressed at 300 MPa, and sintered at1100 °C for 10 h. The density of the sintered samples wasmeasured bythe Archimedes method and the oxygen content was determined byiodometric titration.

The electrical conductivities weremeasured by the four-probe pointmethod at a fixed frequency of 1 kHz. Differential scanning calorimetry(DSC) held in Perkin-Elmer Diamond DSC was used to determinepossible phase transformation. The internal friction Q−1 and shearmodulusMweremeasured in a computer controlled automatic invertedtorsionpendulumwith the forced-vibrationmethodwith themaximumtorsion strain amplitude kept at 1.5×10−5. The thermal expansioncoefficient for GdBaCo2O5+δ specimens was measured using a NetzschDIL 402C dilatometer in air with a heating rate 5 °C/min.

Fig. 1. Arrhenius curves of the conductivity (a denotes experimental curve and b fittedcurve) and DSC measurement (c) for GdBaCo2O5+ δ.

Fig. 3. Resistivity relaxation plots for GdBaCo2O5+ δ at 700 °C for oxidation andreduction steps.

246 W. Liu et al. / Solid State Ionics 192 (2011) 245–247

3. Results and discussion

The extra oxygen content, δ, of the as-air synthesized GdBaCo2O5+ δ

sample is 0.421 and the relative density is about 95%. Fig. 1 shows thetemperature dependence of conductivity and DSC measurement forGdBaCo2O5+ δ. The conductivity undergoes two transitions, bothcompanied by obvious thermal effects. The rapid increase observed atabout 75 °C is usually considered as a insulator-metal (IM) transitionand correlated with the oxygen stoichiometry [2]. However, thetemperature dependence of conductivity does not show a simplemetallic behavior above TIM. The conductivity stillweakly increaseswithtemperature in the temperature range TIM–350 °C, and after thatexhibits metal-like temperature dependence. The second transition ofthe conductivity of GdBaCo2O5+δ occurs around 500 °C. S. Streule et al.[14] also observed this transition at 776 K in PrBaCo2O5.48, which wasascribed to the redistribution of oxygen ion from an ordered todisordered state within the Pr–O planes. Oxygen disorder breaks anideal alternation of octahedral and pyramidal planes, hampering thehole creation.

Fig. 2 gives the shear modulus and internal friction of GdBaCo2-O5.421 with different frequencies at a heating rate of 2 K/min. Twointernal friction peaks are readily observed. Peak 1 around 77 °C hasthe characteristic of first order phase transformation, i.e., peak heightdecreases with frequency increasing while peak position is indepen-dent of frequency. At the same time, the corresponding shearmodulusachieves a local minimum. The temperature of Peak 1 coincides withthat of insulator-metal transition, indicating Peak 1 corresponds to IMtransition. The IM transition is usually associated to a spin statetransition of the Co3+ located in an octahedral environment [14].

Fig. 2. Temperature dependence of the shear modulus M and internal friction Q−1 ofGdBaCo2O5.421 with various frequencies at a heating rate of 2 °C/min.

Since the Co3+ ions with different spin state show different ionicradius, a change of unit cell volume can be expected. Our internalfriction technique reveals well this lattice change. For peak 2 at about200 °C, it is of the relaxation type, having the characteristic that thepeak temperature shifts to higher temperature with the frequencyincreasing. Further it is worth to point out that the peak height hasalso a slight increase with the frequency increasing. The internalfriction peak is related to oxygen motion within the Gd–O planes [15]and the relaxation parameters E=1.00 eV and τo=1.3×10−12 s areobtained.

Similarly to other perovskite, the oxygen content of GdBaCo2O5+δ

decreases with temperature increasing or oxygen partial pressure Po2decreasing, which should change the concentration of charge carrierand further influence the electrical conductivity. Fig. 3 gives represen-tative resistivity relaxation profiles for GdBaCo2O5+δ at 700 °C forreduction and oxidation steps. As can be seen, the oxidation experimentwas remarkably faster than the corresponding reduction process as thegas switched from air to Helium. However, as the gas switched from airto oxygen, the approximate time constant for oxidizing and reducingprocesses were not only short but also nearly equal. These indicate thatthe kinetics of oxygen incorporated into or released from the solid oxideintensely depends on the oxygen partial pressure. Moreover, as show inFig. 4, the oxygen exchange kinetics is also closely related totemperature and the relaxation time decreases with temperatureincreasing, suggesting that the oxygen exchange is a temperature-activated process. In addition, the electrical conductivity monotonicallyincreases with the oxygen partial pressure increasing, which is anindication that holes are the majority charge carriers in GdBaCo2O5+δ.Hole-conductivity in cobaltites is usually related to the presence of Co4+

Fig. 4. Conductivity relaxation plots for GdBaCo2O5+ δ at different temperature foroxidation and reduction steps: O2→He→O2.

Fig. 5. Thermal expansion behavior of GdBaCo2O5+ δ in air.

247W. Liu et al. / Solid State Ionics 192 (2011) 245–247

ions and as in other perovskite systems, the thermally induced chargedisproportionation reaction 2Co3+→Co2++Co4+ provides the mobileCo4+ charge carriers [14]. Therefore, conductivity diminishes withoxygen pressure decreasing is due to the decrease of the oxygen contentand consequently reduction of mean oxidation degree of cobalt ions.

Apart from good electrical performance, a promising SOFC cathodematerial should also meet other requirements like thermal expansionmatch with electrolyte materials. Fig. 5 shows the thermal expansionbehavior of GdBaCo2O5+ δ in air (curve a). It can be seen that theexpansion curve is sigmoid in shape between about 200 and 500 °C.This phenomenon is similar to some cobaltites such as LnCoO3-δ

(Ln, Eu, and Y) [16], but different from that of Ba1-xSrxCo0.8Fe0.2O3-δ

or La1-xSrxCo0.2Fe0.8O3-δ reported in the literatures [17,18]. Typicalexpansion curves of Ba1-xSrxCo0.8Fe0.2O3-δ or La1-xSrxCo0.2Fe0.8O3-δ

show only sudden increases in their slopes, which is generallyattributed to the loss of lattice oxygen due to the reduction of Fe4+

and Co4+ to lower valence states [18]. However, it can be seen fromthe thermogravimetry of GdBaCo2O5+ δ (Fig. 5 curve b) that theoxygen content increases with temperature increasing between 250and 450 °C. That is to say, the sample undergoes an oxidationprogress, which could be related to a change in crystal structure.The average thermal expansion coefficient (TEC) calculated fromthe thermal expansion curve is 21.4×10−6 K−1 in the temperaturerange of 500–900 °C. This relative higher TEC is not suitable used ascathode materials in SOFC with YSZ electrolyte, but might be usedin other low temperature SOFC system with new electrolyte.

4. Conclusions

The electrical conductivity of GdBaCo2O5+ δ monotonicallyincreases with the oxygen partial pressure increasing, indicating thatholes are the majority charge carriers in GdBaCo2O5+ δ. The oxygenexchange is a temperature-activated process and intensely depends onoxygen partial pressure. The conductivity undergoes two transitions,both companied by obvious thermal effects. The conductivitytransition at about 75 °C is well revealed by an internal friction peakwith typical features of first order phase transformation. A largerelaxation internal frictionpeak is also observed at about 200 °C,whichis due to the oxygen hopping within Gd–O plane and the relaxationparameters are E=1.00 eV and τo=1.3×10−12 s. The thermalexpansion experiment shows the average TEC of GdBaCo2O5+ δ isabout 21.4×10−6 K−1 at 500–900 °C.

Acknowledgement

This work was supported by Natural Science Foundation of China(No. 20371045 and No. 10574123) and the Master-Doctoral InitialFoundation of Anhui University of Architecture (2008).

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