Characterisation, conductivity and mechanical properties of the oxygen-ion conductor La0.9Sr0.1Ga0.8Mg0.2O3-x John Drennan,a† Viktor Zelizko,a David Hay,a Fabio T. Ciacchi,a S. Rajendranb and Sukhvinder P. S. Badwal*b aCSIRO, Division ofMaterials Science and T echnology, Private Bag 33, RosebankMDC, Clayton, V ictoria 3169, Australia bCeramic Fuel Cells L imited, 710 Blackburn Road, Clayton, V ictoria 3168, Australia The new oxygen-ion conductor La0.9Sr0.1Ga0.8Mg0.2O3-x has been prepared by conventional solid-state reaction at high temperatures and characterised by X-ray diffraction, scanning and transmission electron microscopy, and conductivity (four-probe dc and impedance) measurements.The room-temperature structure is orthorhombic, space group Pnma (no. 62), with a= 5.5391(7) A° , b=7.8236(12) A° , c=5.5224(7) A° .The material undergoes a phase transition at 445 K to a rhombohedral structure. Mechanical property measurements at room temperature and at 1173 K give average strength measurements of 162±14 MPa and 55±11 MPa respectively. Conductivity and ionic transport number measurements confirm predominantly ionic conduction. The contribution from the grain boundary conductivity is extremely small at temperatures below 673 K.At 1073 K, an ionic conductivity value of 0.12 S cm-1 was recorded in air. It has been reported recently1–4 that the LaGaO3 perovskite temperature. The required amounts of lanthanum oxide substituted at the A and B sites shows good oxygen-ion (>99%, calcined at 1000 °C for 2 h before use), gallium oxide conducting properties at elevated temperature.The most pro- (99.99%), magnesium oxycarbonate (>99%) and strontium mising candidate materials have been shown to be those carbonate (>99%) were mixed and milled together in isopropyl substituted at the A site with Sr and at the B site with Mg. alcohol for 24 h followed by calcination at 1423 K for 4 h. The The highest conductivity has been reported for the composition calcined powder was grey and X-ray diffraction pattern showed La0.8Sr0.2Ga1-yMgyO3-x (y=0.10–0.15).4 The electrolyte is it to be single phase.The oxide powder was milled again in commonly referred to as LSGM. The material is reported to isopropyl alcohol for 24 h, dried and pressed into bar shapes be stable in both reducing and oxidising atmospheres up to for four-probe dc and impedance measurements, or disc shapes 1223 K and shows a reported ionic conductivity of for mechanical strength measurements, and sintered at > 0.1 S cm-1 at 1073 Kand an ionic transport number of close 1723 K for 15 h (heating and cooling rates of 300°C h-1).The to unity. Other materials with high ionic conductivity, such as sintered discs were darkish grey but had a density doped ceria and bismuth oxide, are unstable in reducing (6.58 g cm-3)>98.5% of the theoretical.environments and develop substantial electronic conductivity. The sintered and polished specimens were examined with a The thermalexpansion coefficient and the oxygen ionic conduc- scanning electron microscope. Detailed characterisation of tivity domain (temperature and oxygen partial pressure range) sintered samples was undertaken using a combination of are close to those of stabilised zirconias.The perovskite-based analytical electron microscopy (ATEM) and X-ray diffraction materials have often been discussed5,6 as possible ionic conduc- techniques. ATEM was carried out on both crushed and tors with the tantalising prospect of being able to engineer ion beam thinned specimens using a Philips CM30 series substitution of aliovalent cations onto both the A and B sites electron microscope.Energy dispersive X-ray spectra were (ABO3) with a view to introducing a variety of vacancy recorded using an EDAX 9900 system and all micrographs, schemes and enhanced oxygen-ion conductivity.Until the diffraction patterns and spectra were recorded with the micro- recent report by Ishihara et al.1 on LSGM, the results have scope operating at an accelerating voltage of 300 keV. In the been disappointing. With this discovery the renewed interest case of the ion beam thinned specimens it was found necessary in this class of materials may prove to be a fruitful area to coat the sample with a thin layer of carbon to avoid of research.charging problems. A Siemens D500 diffractometer was used Apart from ionic conductivity measurements and some to collect X-ray diffraction patterns using graphite-monochro- information on the thermal expansion behaviour (thermal mated Cu-Ka radiation. XRD data were also collected at expansion coefficient a=10-5 K-1 3) only limited physical 483 K using a locally constructed temperature stage for the property data are available for LSGM.Moreover, there is D500 diffractometer. Refinement of structural parameters was some confusion over the crystallographic characterisation of carried out by Rietveld methods using the program the material. In this paper we attempt to address these WYRIET 3.7 deficiencies by reporting mechanical property data, conduc- Differential thermal analysis (DTA) measurements were tivity measurements (both four-probe dc and impedance) and made with a Stanton Redcroft Thermochemical Analyser TMA crystallographic characterisation which was obtained using a series 793.The heating rate used was 10°C min-1. combination of analytical electron microscopy and X-ray Four-probe dc conductivity measurements were performed diffraction techniques. in air as a function of temperature (673–1273 K) at 10–25 K intervals during both heating and cooling cycles and as a Experimental function of time at 1123 and 1273 K on different specimens.The specimens were ca. 20–22 mm long with linear conduction The powder of composition La0.9Sr0.1Ga0.8Mg0.2O2.85 was areas of ca. 0.21 cm2. The details of the experimental set-up prepared by conventional solid-state reaction at a high have been described in a previous publication.8 For impedance measurements, specimens were cut from bars and had dimen- † Present Address: Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia. sions of a=5.3 mm, b=7.5 mm and thickness t=3.9 mm.J. Mater. Chem., 1997, 7(1), 79–83 79Table 1 Room-temperature strength of LSGM samplesa sample tb/mm wc/mm load/N strength/MPa 1 1.484 16.32 279 138 2 1.510 16.31 339 162 3 1.495 16.31 335 164 4 1.263 15.60 265 183 5 1.248 15.61 233 165 6 1.257 15.64 228 159 aAverage strength=162±14 MPa. bt=thickness. cw=diameter. Table 2 High-temperature strength of LSGM samplesa sample t/mm w/mm load/N strength/MPa 1 1.478 15.13 137 67 2 1.513 14.98 125 58 3 1.496 14.97 126 60 Fig. 1 Optical micrograph of an LSGM sample 4 1.548 14.98 112 50 5 1.540 15.06 143 64 6 1.471 15.02 72 35 7 1.484 15.00 104 50 aAverage strength=55±11 MPa at 1173 K. Platinum paste electrodes sandwiched between platinum mesh were used for current collection. Impedance measurements were performed over the temperature range 573–723 K and the frequency range 5 Hz–5 MHz (with a Hewlett Packard 4192A impedance analyser) in air at 25 K intervals. Transport number was measured by constructing a galvanic cell with platinum paste electrodes and exposing one side of the cell to air and the other side to controlled H2–H2O gas mixtures at different temperatures.Flexural strength measurements were made at room temperature and at 1173 K. Disc-shaped sintered specimens (dimensions are presented in Tables 1 and 2) were used. Prior Fig. 2 Rietveld pattern refinement of LSGM as the orthorhombic to testing, both surfaces of the discs were ground using a (Pnma) structure. The observed XRD trace is shown, together with diamond wheel and one of the surfaces was polished to a 1 mm the theoretical trace using refined parameters, and a difference trace finish using diamond paste.This is to ensure that the tensile is shown below. Expected peak positions from the model structure surface is free from any pre-existing flaws. The loading con- are shown as bars. figuration for the test was similar to that described in the ASTM F394-78 9 publication.In the room-temperature case, b=7.8236(12) A° and c=5.5224(7) A° . Refinement converged to the three support balls were equally spaced on a circle of Bragg R=5.1%. Refined atomic coordinates are given in 12.098 mm diameter with the loading ram of 3.946 mm diam- Table 3. eter. At 1173 K, support pins with hemispherical ends and The room-temperature symmetry was further confirmed by having diameters of 3 mm were used.The pins were spaced analytical transmission electron microscopy (ATEM). equally on a circle of diameter 11.493 mm and the loading ram Convergent-beam electron diffraction patterns recorded from was 3.95 mm in diameter. individual grains of the LSGM showed extra reflections which are not consistent with a primitive cubic cell.Fig. 3 shows an Results and Discussion indexed orthorhombic pattern ([100]) based on the unit cell described above. To further confirm that the material was in The material was easily fabricated into dense bodies. The fact homogeneous, energy dispersive X-ray spectra were microstructure of a polished and etched specimen is shown in recorded from a number of grains and an example is shown Fig. 1. In general, very few but uniformly distributed pores in Fig. 4. Clearly a solid-state reaction has taken place and were observed in the microstructure. Clearly a bimodal grain La, Sr, Ga and Mg were detected; composition variations size distribution was observed in the microstructure. Small between grains were insignificant. Quantitative analysis grains were in the 3–5 mm and large grains in the 15–25 mm determined using the peak intensities gave values close to the size range.The room-temperature X-ray diffraction patterns nominal composition but difficulties with overlapping peaks of the prepared samples used in this study showed a basic perovskite-related structure. However, on closer examination Table 3 Refined fractional atomic coordinates with estimated standard it is clear that some reflections showed shoulders and splitting deviations in parentheses representing a reduction in symmetry.Goodenough and Feng3 reported that the La0.9Sr0.1Ga0.8Mg0.2O2.85 phase was cubic atom 103x/a 103y/b 103z/c perovskite, whilst Ishihara et al.1 have reported an orthorhombic structure for this material. In this study the powder patterns La 9998(14) 2500 -18(36) Sr 9998(14) 2500 -18(36) were indexed using the orthorhombic cell and Rietveld refine- Ga 0 0 5000 ments carried out on this basis.The results of Rietveld full pat- Mg 0 0 5000 tern refinement are shown in Fig. 2. The atomic model which O1 4380(119) 2500 584(129) gave the best fit to the data was orthorhombic, space group O2 2409(127) 504(32) 7522(174) Pnma (no. 62), with refined cell dimensions a=5.5391(7) A° , 80 J. Mater. Chem., 1997, 7(1), 79–83Fig. 3 The [100] diffraction pattern of an LSGM specimen Fig. 5 Bright-field micrograph recorded from part of a grain of LSGM showing the strain and massive twinning which is a feature of this material. Inset in the micrograph are microdiffraction patterns recorded from the regions directly underneath the patterns.Clearly a twin relationship exists. in the high-temperature trace, part of which is shown in Fig. 6, Fig. 4 A portion of the energy dispersive X-ray spectrum showing the where peaks appear as a multiplet. This is characteristic of a presence of La, Sr, Ga and Mg in the grain of an LSGM specimen change to rhombohedral (R3c) symmetry, where these peaks appear as a triplet, as observed for the phase transition at and absorption effects introduced some uncertainties into these 418 K in LaGaO3.12 It should be noted that Petric et al.4 calculations.observed extra reflections for LSGM in the electron diffraction A further observation of the microscopy revealed that the pattern. They described these reflections as superstructure grains of LSGM at room temperature contained contrast reflections resulting from the formation of microdomains.In features consistent with twinning. Microdiffraction patterns the work reported in this study, although such reflections were confirmed that these domains are associated with twinning observed, these could be fully indexed using the orthorhombic and this is illustrated in Fig. 5. This phenomenon has been cell structure. observed in electron microscopy studies of the related perov- The consequence that this phase transition has on the skite LaGaO3.10 It should be noted that extensive crystallo- suitability of LSGM as an electrolyte material is unknown but graphic studies have been made of LaGaO3 and the material the amount of strain evident in the micrographs (Fig. 5) has an orthorhombic structure at room temperature with lattice parameters a=5.5232(5) A° , b=7.776(2) A° , c= 5.4925(7) A° and the space group Pnma (no. 62) satisfies the diffraction evidence. In addition, LaGaO3 is known11 to undergo a phase transition at 418 K in which the system transforms from orthorhombic symmetry to rhombohedral symmetry and the consequent readjustment gives rise to elimination of twins.DTA results obtained from the LSGM specimens examined in this work revealed that an endothermic peak was detected at 445 K, slightly higher than that reported for pure LaGaO3 (418 K), but certainly indicating a phase transition. It is a logical assumption, therefore, that the twin boundaries observed in LSGM are a result of a possible phase transition similar to that observed in LaGaO3.To further confirm the DTA results, high-temperature X-ray diffraction was employed to examine for any evidence of this phase transition. Indeed, evidence of a phase change is seen in the XRD data at 483 K. The group of peaks at 2h#32° is seen as a doublet in the orthorhombic (Pnma) room-temperature struc- Fig. 6 Expansion of the peak structure near 32° showing the roomtemperature (a) and 483 K (b) XRD traces ture.However, further splitting of this group of peaks is seen J. Mater. Chem., 1997, 7(1), 79–83 81Table 5 Activation energy data for LSGM, 9 mol% Sc2O3–ZrO2 and suggests that thermal cycling through the transition point may 9 mol% Y2O3-ZrO2 be deleterious to mechanical properties. The results of strength measurements on LSGM samples at (a) From four-probe dc conductivity data: room temperature and at 1173 K are presented in Tables 1 Ea/kJ mol-1 and 2.The average strength determined was 162±14 MPa at room temperature. Specimens dimensions, fracture loads and 400–450°C 850–1000°C determined strength for 1173 K strength tests are given in Table 2. At this temperature the average strength determined La0.9Sr0.1Ga0.8Mg0.2O3-x 109±3 64±2 9 mol% Sc2O3–ZrO2 130±2 72±3 for LSGM was 55±11 MPa.The flexural strength of these 9 mol% Y2O3–ZrO2 107±2 81±3 materials is significantly lower in comparison to the zirconiabased electrolytes presently in use. It is worth mentioning here (b) From impedance dataa (300–450 °C): that the mechanical strength of fully stabilised zirconia is in the vicinity of 300 MPa at room temperature and 120 MPa at Ea/kJ mol-1 1273 K.13 For yttria–tetragonal zirconia ceramics, flexural strengths of around 1000 MPa and 350–400 MPa have been La0.9Sr0.1Ga0.8Mg0.2O3-y Rv Rgb Rtotal measured at room temperature and 1273 K respectively.13 before annealing 107±1 97±2 105±1 Fig. 7 shows Arrhenius plots for the conductivity data of after 1000 °C anneal, 5000 min 105±1 96±2 104±1 LSGM, 9 mol% Sc2O3–ZrO2 and 9 mol% Y2O3–ZrO2.Clearly after 850 °C anneal, 5000 min 106±1 98±2 105±1 the conductivity of LSGM is higher than that of 9 mol% Y2O3–ZrO2 , but over the temperature range 1073–1273 K the aRv=volume (or lattice) resistivity, Rgb=grain boundary resistivity, ionic conductivity values for both materials are comparable.Rtotal=Rv+Rgb . Table 4 gives ionic conductivity values for several high-conductivity electrolyte materials. At 1073 K, the ionic conductivity of Sm2O3-doped ceria is slightly lower than that of LSGM. The activation energy values for conduction are given in Table 5. At high temperatures (1123–1273 K), the activation energy for LSGM was lower than for both 9 mol% Sc2O3–ZrO2 and 9 mol% Y2O3–ZrO2 materials.Conductivity data as a function of time at 1123 and 1273 K for LSGM are displayed in Fig. 8. Only a slight decrease in the conductivity was observed with time at 1273 K. At 1123 K, the effect of time on conductivity was insignificant. This absence of any ageing process clearly indicates that the phase assemblage or the structure of the material is stable.This was further confirmed by transmission electron microscopy of the annealed (1273 K, 5000 min) specimen. Impedance measurements on LSGM specimens over the 573–723 K range show a very low contribution from the grain Fig. 8 Dc conductivity for LSGM as a function of time at 1123 (a) and 1273 K (b) boundary resistivity (Fig. 9). A slight decrease in the volume resistivity as a result of annealing at 1273 K could not be explained by the conductivity or activation energy data or by the detailed microstructural analysis.In order to determine ionic transport number, small fuel cells were constructed with Pt air and fuel electrodes and LSGM as the electrolyte. Fig. 10 shows the results of open circuit voltage measurements at several temperatures for air vs.H2–H2O mixture. In all cases the measured voltage was close to the theoretical (ionic transport number close to unity) indicating that the material is mainly an ionic conductor. Fig. 7 Arrhenius plots (four-probe dc data) for LSGM (#), 9 mol% Sc2O3–ZrO2 (%) and 9 mol% Y2O3–ZrO2 (') Table 4 Conductivity data for some high-conductivity oxygen-ion conducting electrolytes s/S cm-1 s/S cm-1 system (1073 K) (1273 K) ref.LSGM 0.121 0.316 (ZrO2)0.91 (Y2O3)0.09 0.046 0.166 14 (ZrO2)0.91 (Sc2O3 )0.09 0.109 0.306 14 (CeO2)0.82 (Gd2O3)0.18 — 0.235 15 (CeO2)0.80 (SmO1.5)0.20 0.096 0.25 16 (Bi2O3)0.80 (Er2O3)0.20 0.37a — 17 (Bi2O3)0.80 (Nb2O5)0.20 0.19a — 18 Fig. 9 Impedance plots at 623 K in air for LSGM before (a) and after (b) annealing at 1273 K for 5000 min aAt 973 K. 82 J. Mater. Chem., 1997, 7(1), 79–83specimen preparation and to Dr. K. Foger for reviewing this manuscript. References 1 T. Ishhihara, H. Matsuda and Y. Takita, J. Am. Chem. Soc., 1994, 116, 3801. 2 T. Ishihara, H. Matsuda and Y. Takita, Solid State Ionics, 1995, 79, 147. 3 M. Feng and J. B. Goodenough, Eur. J. Solid State Inorg. Chem., 1994, 31, 663. 4 A.Petric, P. Huang and A. Skowron, Proc. 2nd Eur. SOFC Forum, ed. B. Thorstensen, Druckerei J Kinzel, Go�ttingen, Germany, 1996, Fig. 10 Open-circuit voltage [measured (#), calculated (%)] for Pt, pp. 751–760. air|LSGM|Pt, H2–2%H2O cell as a function of time. The solid line 5 J. B. Goodenough, A. Manthiram and J-F. Kuo, Mater. Chem. represents the calculated cell voltage. Phys., 1993, 35, 221. 6 A. F. Sammells, R. L. Cook, J. H. White, J. J. Osborne and R. C. MacDuff, Solid State Ionics, 1992, 52, 111. Conclusions 7 M. Schneider, Program WYRIET 3, version 3, D-8134 Pocking, West Germany, 1992. At 1273 K, the ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-x 8 S. P. S. Badwal, F. T. Ciacchi and D. V. Ho, J. Appl. Electrochem., is higher by a factor of two compared to ZrO2 doped with 8 1991, 21, 721.mol% Y2O3 ; however, it was similar to that of ZrO2 doped 9 ASTM Designation: F 394–78. with 9 mol% Sc2O3. The major drawback of these materials 10 M. Sundberg, P-E. Werner, M. Westdahl and K. Mazur, Mater. as potential electrolytes for use in solid oxide fuel cells or other Sci. Forum, 1994, 166–169, 795. 11 H. M. O’Bryan, P. K. Gallagher, G. W.Berkstresser and similar applications is the high cost of gallium compounds, C. D. Brandle, J.Mater. Res., 1990, 5, 183. related to its scarcity, and the low mechanical strength of 12 Y. Wang, X. Liu, G.-D. Yao, R. C. Lieberman and M. Dudley, LSGM, especially at the fuel cell operating temperatures. It is Mater. Sci. Eng. A, 1991, 132, 13. unlikely that such materials can be used in electrolyte-sup- 13 V. Zelizko, unpublished work. ported designs for solid oxide fuel cells or in applications 14 S. P. S. Badwal, F. T. Ciacchi, J. Drennan and S. Rajendran, to where they must play some role in the structural design of a be published. 15 T. Kudo and H. Obayashi, J. Electrochem. Soc., 1976, 123, 415. device. A more likely application would be in their use as thin 16 K. Eguchi, T. Setoguchi, T. Inoue and H. Arai, Solid State Ionics, coatings on an electrode substrate. Alternatively these materials 1992, 52, 165. may find use in sensors where the current-carrying capacity of 17 M. J. Verkerk, K. Keizer and A. J. Burggraaf, J. Appl. Electrochem., the device is not important and the materials are used in small 1980, 10, 81. quantities. 18 T. Takahashi, H. Iwahara and T. Esaka, J. Electrochem. Soc., 1977, 124, 1563. The authors are thankful to Dr. S. P. Jiang for determining the ionic transport number, Miss Kristine Giampietro for Paper 6/04563D; Received 1st July, 1996 J. Mater. Chem., 1997, 7(1), 79–83