Activity of SrO in La1−xSrxMnO3−y (x=0.065, 0.10, 0.15 or 0.20) by a solid state EMF method

Journal of Alloys and Compounds 322 (2001) 113–119 L www.elsevier.com / locate / jallcom Activity of SrO in La 12x Sr x MnO 32y (x50.065, 0.10, 0.15 or 0.20) by a solid state EMF method S. Arul Antony a , K. Swaminathan b , K.S. Nagaraja a , O.M. Sreedharan b , * b a Department of Chemistry, Loyola College, Chennai-34, India Thermodynamics and Kinetics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India Received 26 September 2000; received in revised form 7 February 2001; accepted 14 February 2001 Abstract Thermodynamic activity of SrO, a SrO , in lanthanum manganite doped with SrO with the stoichiometry La 12x Sr x MnO 32y (where x50.065, 0.10, 0.15 or 0.20) was measured as a function of temperature over the range of |750–1000 K by employing an EMF technique with CaF 2 or SrF 2 as the fluoride ion conducting electrolyte under an atmosphere of oxygen at unit fugacity. When Pt / SrO,SrF 2 ,O 2 was used as the reference electrode for the test electrode Pt / 0.15 LSM, SrF 2 ,O 2 , sintered SrF 2 was used as the electrolyte. In all other galvanic ] cells, both the test and reference electrodes contained LSM with different dopant concentrations. The SrO potential DGSrO , log a SrO and the corresponding activity coefficients (gSrO ) for the four solid solutions were determined and log a SrO was found to exhibit decreasing ] negative deviation from ideality with increasing SrO content. The DGSrO for these compositions could be represented as 24.81–0.03360T (K) for 0.065 LSM, 25.68–0.02818T for 0.10 LSM, 29.84–0.01950T for 0.15 LSM and 211.42–0.01360T for 0.20 LSM in terms of kJ mol 21 .  2001 Elsevier Science B.V. All rights reserved. Keywords: Lanthanum strontium manganite; Thermodynamic activity 1. Introduction The perovskite type inter-oxide LaMnO 3 doped mainly with SrO (designated as LSM with the typical composition La 0.8 Sr 0.2 MnO 32y ) is widely accepted as the cathode material for the high temperature solid oxide fuel cells (SOFCs) [1,2]. Though there are several other criteria such as high electronic conductivity in the oxidant atmosphere, resistance to sintering (to retain porosity) and high thermal expansion co-efficient matching with other SOFC components, the compatibility with solid electrolyte-interconnect in terms of activity of strontia and high temperature thermodynamic stability in an oxidizing atmosphere of the cathode appeared to be the major requirements that guided the choice of LSM as the cathode material. The equilibrium dissociation pressure of LaMnO 3 into La 2 O 3 and MnO at 1273 and 1473 K was first reported by Nakamura et al. [3] by employing isothermal controlled atmosphere (Po 2 )TG. Following this work Sreedharan et al. [4] reported the dissociation pressure of LaMnO 3 by employ*Corresponding author. E-mail address: [email protected] (O.M. Sreedharan). ing a solid oxide electrolyte galvanic cell technique to measure the oxygen potential of the electrode LaMnO 3 / MnO / La 2 O 3 over the temperature range 1064–1308 K. Using the electrical conductivity to monitor the dissociation temperature of LaMnO 3 , Kamegashira et al. [5] reported the dissociation pressures of LaMnO 3 as a function of temperature over the interval 1173–1473 K. Hildrum et al. [6] resorted to isothermal coulometric dissociation of undoped LaMnO 3 as well as doped 0.20 LSM to determine the dissociation pressures at different temperatures namely 1273, 1323 and 1373 K. There was yet another measurement of the dissociation pressure of undoped LaMnO 3 at 1273 K by Kuo et al. [7] using an electrical conductivity measurement by varying the PO 2 of the sample environment. The oxygen stoichiometry of both SrO doped and undoped LaMnO 3 in oxidizing environments are widely different from those under an inert or reducing atmosphere [8–10]. A more useful parameter would be the activity of SrO in LSM under oxidizing atmospheres which would enable a realistic evaluation of the compatibility of LSM with the electrolyte and the interconnect with respect to transfer of SrO from region of higher to lower SrO potential. Hence the following in- 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01215-4 114 S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 vestigation was undertaken to experimentally determine a SrO as a function of dopant concentration in the range of 0.065–0.20. 2. Experimental 2.1. Materials High purity lanthanum acetate (better than 99%, IRE, India) was decomposed in air at 1173 K to yield anhydrous La 2 O 3 . Reagent grade MnCO 3 (better than 99%, Fischer, India) was reduced in a stream of H 2 gas at 1023 K for 6 h to yield MnO. Reagent grade SrCO 3 (better than 99.9%, Merck, India) served as the source for strontia by heating at 1423 K for 16 h followed by rapid cooling in an inert gas cover to avoid the formation of superoxide. Powders (purity better than 99.99%, Aldrich, USA) were used for the preparation of sintered discs of SrF 2 . Single crystal CaF 2 (BARC, Trombay, India) was used as an electrolyte in the galvanic cells II–V. Sintered discs of polycrystalline SrF 2 were used as an electrolyte for cell I and for counterchecking EMF results of cell II by replacing CaF 2 (single crystal) as the electrolyte. These sintered discs were prepared by compacting finely powdered SrF 2 into cylindrical discs of dimensions 15 mm diameter and 5 mm thickness at a pressure of 180 MPa. These were then sintered in a zirconia boat under a cover of high purity argon (99.999%) by heating to 1693 K at a heating rate of 5 K / min. Subsequently these were cooled in argon, at 3 K / min, to 1523 K where they were held for 12 h to minimize vaporization which would otherwise lead to porosity. The chemicals used for the wet chemical synthesis of LSM were reagent grade ethylene glycol (Aldrich) citric acid (Merck) and nitric acid (Fischer). ods. In this hybrid method, the required quantities of La 2 O 3 , SrCO 3 and MnO were separately dissolved in 50% excess of warm 1:3 HNO 3 . These solutions were then carefully added to the hot ester which in turn was prepared by heating an equimolar mixture of citric acid and ethylene glycol. The heating was accompanied by constant stirring until the temperature was held at 365 K for 1–2 h. By heating the syrupy liquid, which consisted of a mixture of the ester and nitrate in the molar ratio of 10:1 with an excess of citric acid, the temperature was gradually raised to 620 K and maintained for 3–4 h. The color of the viscous solution gradually changed from yellow to red followed by setting to a brown colored, transparent glassy gel. On further heating at 820 K for 1 h the gel charred into a powdery resinous mass and finally a black solid mass (referred to as ‘precursor’) was obtained. Subsequently the precursor powder was compacted into cylindrical pellets as mentioned earlier. These pellets were then calcined at 900 K for 1 h and subjected to final sintering at 1120 K for 4 h. 2.2.2. EMF measurements The anode pellets were prepared by compacting equimolar mixture of SrO and SrF 2 at room temperature into cylindrical pellets of 100 mm diameter and 2–3 mm thickness by applying a pressure of 100–120 MPa under a cover of an inert gas. LSM of each composition was intimately ground with SrF 2 in the molar ratio 3:1 and compacted into pellets. High purity oxygen, dried by passing through columns of suitable drying agents, was used as the cover gas. An open-cell-stacked-pellet assembly, with either single crystal CaF 2 or sintered SrF 2 as electrolyte, was used. The galvanic cell configurations studied were Pt, SrO, SrF 2 , O 2 u SrF 2 u O 2 , SrF 2 , La 0.85 Sr 0.15 MnO 32y , Pt (I) 2.2. Procedure 2.2.1. Synthesis of LSM Two routes of synthesis, ceramic and hybrid wet chemical methods were adapted. Synthesis of 0.2 LSM was attempted by the former route. For this purpose SrCO 3 , La 2 O 3 and MnO were compacted in stoichiometric proportions into pellets of 10 mm diameter and 3–5 mm thickness under a pressure of 100 MPa. These pellets were then subjected to heat treatment in stages at different temperatures in the range 1073–1473 K with intermittent grinding and compaction and by varying the local environment from oxygen to vacuum and finally to ambient air until the product was identified to be phase pure La 0.8 Sr 0.2 MnO 32y as confirmed by powder X-ray diffraction (XRD) techniques within its 5 mass percent threshold for detection of impurity phases. The hybrid wet chemical method adapted here for the synthesis of 0.065, 0.10, 0.15 and 0.20 LSMs was a combination of polymeric-gel and autocombustion meth- Pt,La 0.85 Sr 0.15 MnO 32y , SrF 2 , O 2 u CaF 2 u O 2 , SrF 2 , La 0.935 Sr 0.065 MnO 32y , Pt (IIa) Pt, La 0.85 Sr 0.15 MnO 32y , SrF 2 , O 2 u SrF 2 u O 2 , SrF 2 , La 0.935 Sr 0.065 MnO 32y , Pt (IIb) Pt, La 0.8 Sr 0.2 MnO 32y , SrF 2 , O 2 u CaF 2 u O 2 , SrF 2 , La 0.935 Sr 0.065 MnO 32y , Pt (III) Pt,La 0.85 Sr 0.15 MnO 32y ,SrF 2 ,O 2 u CaF 2 u O 2 , SrF 2 , La 0.9 Sr 0.1 MnO 32y , Pt (IV) Pt, La 0.8 Sr 0.2 MnO 32y ,SrF 2 , O 2 u CaF 2 u O 2 , SrF 2 , La 0.9 Sr 0.1 MnO 32y , Pt (V) A Pt-10% Rh / Pt thermocouple (Type S) calibrated at S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 freezing points of Sn, Bi, Zn, Sb and Ag was used for temperature measurements by locating the hot junction in the vicinity of the stacked pellets in the uniform zone of a bifilar wound tubular furnace. High purity oxygen, which was dried by passing through appropriate drying columns provided an environment of unit fugacity to the electrode pellets by purging through the cell at a slow rate of 5 dm 3 / h. The reversibility of the EMF was checked by a 10 mol% variation in the relative amount of phases used in the test electrode and the different runs were designated by different letters of the alphabets. In the case of galvanic cell I, sintered discs of SrF 2 were used as fluoride ion conductors for reasons of compatibility with SrO of unit activity (see Section 3). Generally single crystal CaF 2 was experimentally found to be compatible with other electrodes of lower SrO concentrations (x#0.20). However in order to verify the compatibility of CaF 2 with SrO present in solid solution as LSM, two identical cells, one based on CaF 2 and the other based on sintered SrF 2 , were independently used as solid electrolytes for galvanic cell of config. II. In addition, to check the internal consistency, galvanic cell V was assembled and studied at fixed temperatures in the vicinities of 900 and 1000 K. The other details of the fluoride electrolyte EMF measurements are as reported elsewhere [11]. 115 standardizing the conditions for 0.2 LSM, other compositions namely 6.5, 10 and 15 mol.% SrO doped LaMnO 3 were prepared by similar routes. The products were found to exhibit gradual changes in structure from orthorhombic to hexagonal–rhombohedral [13,14]. The XRD patterns of the four doped samples along with the undoped LaMnO 3 are given in Fig. 1. The precursor in the case of pure LaMnO 3 alone was heated to 1473 K in air (instead of 1120 K) and quenched in order to synthesize orthorhombic LaMnO 3 . For the sake of brevity, only the lattice parameters of the undoped and 0.2 strontia doped LaMnO 3 are compared here with the literature values even though those of the intervening compositions namely 0.10 and 0.15 of SrO also showed similar agreement with the reported values for the corresponding compositions. The X-ray pattern of undoped inter-oxide conformed closely to the orthorhombic structure with the space group Pbnm reported in the literature [13,14] and its lattice parameters were a5553.5, b5769.5 and c5573.5 pm. In the case of La 0.8 Sr 0.2 MnO 32y the structure was found to conform to nearly rhombohedral with the space group R3¯c with the lattice parameters a5547.2 pm and a 560.998. The other structures were intermediate and are omitted here for brevity. 3.2. EMF measurements 3. Results and discussion 3.1. Synthesis The conventional ceramic method for the synthesis of strontia doped LaMnO 3 (LSM) did not yield phase pure LSM, even after heating at 1273 K in air–O 2 for 9 days in three stretches with intermittent phase analysis by XRD. Though the amount of LSM increased with each intermittent grinding and heating, detectable amounts of La 2 O 3 and SrMn 2 O 62x were present even after 9 days. The final heating at 1473 K under vacuum (10 MPa) followed by heating in air at the given temperature, finally yielded La 0.8 Sr 0.2 MnO 32y with impurities below the threshold of detection by XRD. The above trials were done exclusively for synthesizing 0.2 LSM. The products of the hybrid polymeric gel–cumbustion method were found to be single phase without recourse to temperatures higher than 1123 K even for 0.2 LSM. In this hybrid polymeric method, the gelation process ensures homogeneous distribution of cations in the gel matrix thereby precluding segregation of the divalent and trivalent cations. While the gelation step was aided by the presence of both ethylene glycol and citrate, excess citric acid functioned as a fuel and the suitable excess of nitrate as the oxidant triggering auto combustion and hence avoiding excessive heating. The gelation process could be followed by the color changes, corresponding to the dehydration and decarboxylation reactions of excess citric acid [12]. After The EMF results from cell I listed in Table 1 are shown as a plot of EMF against temperature in Fig. 2. Likewise, the results from cells II to IV, which are summarized in Table 2, are plotted in Fig. 3. In the case of cell II EMF points obtained by sintered SrF 2 as an electrolyte are distinguished from those with single crystal CaF 2 as given in the legend. Least squares regression analyses were carried out on different sets of EMF data so as to generate straight line fits in the form of two term expressions which are listed in Table 3 along with the temperature range and precision of respective experiments. The procedure adapted for processing the EMF results for deriving the SrO activity data is illustrated using the least squares expression EI 60.13 (mV) 5 51.66 1 0.10106T (1) obtained for cell I. For two faraday of electricity the two half cell reactions for cell I could be represented as follows At the anode SrO (s) 1 2F → SrF 2 (s) 1 1 / 2 O 2 (g) 1 2e 2 (2a) At the cathode SrF 2 (s) 1 1 / 2 O 2 (g) 1 2e 2 → [SrO] LSM 1 2F 2 Thus the overall virtual cell reaction for two faraday of electricity is [SrO] s → [SrO] LSM (2) The standard Gibbs energy change DG for reaction (2) is S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 116 Fig. 1. XRD patterns of the orthorhombic strontia doped and undoped lanthanum manganite La 12x Sr x MnO 32y . identical with the relative partial molar Gibbs energy of solution of SrO in La 0.85 Sr 0.15 MnO 32y which is computed from the familiar Nernst equation ] DGSrO( 0.15 LSM ) 5 2 2FEI Likewise the DG for cell II could be equated to the strontia potentials of the two electrodes as given by ] ] DGII 5DGSrO( 0.065 ) 2DGSrO( 0.15) LSM (3) ] As is well known DGSrO is nothing but the strontia potential RT ln a SrO . Since pure SrO solid is chosen as the standard state, the activity coefficient gSrO could be derived from the relation a 5 g x. Prior to the perusal of the derived data on the strontia potentials and the activities, it is essential to evaluate the reliability of the EMF results presented in the Tables 1–3. Sintered discs of SrF 2 were used as the fluoride ion conducting electrolyte in place of single crystal CaF 2 to ensure compatibility with pure SrO of unit activity. These S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 117 Table 1 EMF results of galvanic cell I Run T (K) E (mV) Run T (K) E (mV) Run T (K) E (mV) A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 720.4 751.5 783.0 815.0 844.6 875.7 904.0 935.8 974.3 998.0 1019.8 1048.3 124.3 127.6 130.8 134.1 137.0 140.3 143.2 146.4 150.1 152.5 154.7 157.1 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 731.9 766.9 798.5 831.4 858.5 891.1 923.7 962.1 979.2 1007.0 1027.7 125.5 129.1 132.3 135.7 138.5 141.9 145.1 148.9 150.6 153.4 155.5 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 741.1 772.5 804.4 839.8 867.5 899.0 930.9 969.5 984.1 1011.9 1038.1 126.4 129.7 133.0 136.5 139.4 142.7 145.9 149.6 151.1 153.9 156.6 discs of SrF 2 were nearly translucent and were of density .92% theoretical as seen from the physical dimensions. Pycnometric density measurements using dibutylphthalate were avoided to eliminate organic contamination. If pure SrO comes into contact with CaF 2 the electrode–electrolyte reaction SrO 1 CaF 2 → SrF 2 1 CaO (4) would proceed spontaneously, since the DG8 4 at 1000 K computed from thermochemical table [15] is a large negative value of 224.8 kJ hence the use of SrF 2 ensures reliability of the EMF results listed in Table 1. If the single crystal CaF 2 were to be used in the temperature range up to 1000–1100 K, the activity of SrO, a SrO , in LSM should be lower than about 0.05. With a typical electrode bearing 0.15 of SrO in solid solution single crystal CaF 2 as well as sintered SrF 2 were in- Fig. 2. Temperature dependence of reversible EMF of cell I. dividually used as electrolytes and were found to give EMF values that could be fitted to a single least squares expression demonstrating the compatibility of CaF 2 with Table 2 EMF results of galvanic cells IIa, IIb, III and IV Run T (K) E (mV) Run T (K) E (mV) II b A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 798.0 832.0 862.0 885.5 916.2 942.5 967.4 997.0 813.0 847.0 876.0 903.1 931.0 956.4 982.2 32.7 34.9 37.1 39.3 41.6 43.5 45.7 47.8 33.8 36.0 38.2 40.4 42.7 44.6 46.9 II a A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 B1 B2 B3 B4 B5 B6 B7 B8 B9 745.0 772.0 798.4 826.2 854.2 880.2 907.6 933.5 959.7 989.5 759.0 786.0 812.0 840.2 868.4 895.0 910.4 947.2 974.2 28.6 30.0 31.8 34.0 35.7 37.8 39.7 41.6 44.0 45.9 29.2 30.9 32.9 34.8 36.8 38.6 40.5 42.8 44.9 III A1 A2 A3 A4 A5 A6 A7 A8 A9 744.3 773.6 797.1 825.5 859.0 888.7 916.0 946.0 965.0 44.4 46.8 49.5 52.0 54.5 57.2 60.0 64.0 68.7 B1 B2 B3 B4 B5 B6 B7 B8 B9 759.5 786.0 811.0 837.4 873.2 902.1 937.0 953.0 986.0 45.6 48.1 50.7 53.2 56.1 58.6 62.1 65.4 71.5 IV A1 A2 A3 A4 A5 A6 A7 736.5 780.5 827.2 869.5 910.7 956.7 1004.5 12.0 13.8 15.9 17.7 19.8 21.9 23.9 B1 B2 B3 B4 B5 B6 759.7 805.5 848.0 886.0 933.0 979.5 12.9 14.8 16.8 18.8 20.9 22.8 S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 118 squares expressions from Table 3. Having established the reliability of the present experimental result, the derived ] values of DGSrO and log a SrO computed as mentioned earlier are listed in Table 5. The activity co-efficients at two different temperatures 1000 and 1273 K for the four compositions are given in Table 6. The smooth variation of log gSrO at 1000 and 1273 K as a function of SrO concentration in LSM is shown in Fig. 4. The data are extrapolated to 1273 K which is the normal temperature of operation of SOFCs. Thus a value of 0.33 for gSrO and 0.066 for a SrO in La 0.8 Sr 0.2 MnO 32y could be obtained by extrapolating the relevant data from Tables 5 and 6 to 1273 K. This value of 0.066 is much lower than 0.6 required for the formation of SrY 2 O 4 [16]. Thus the present activity data help in assessing the compatibility of LSM with yttria bearing YSZ solid electrolyte in SOFCs. Similar measurements of a SrO in the interconnect material namely doped LaCrO 3 would be useful in designing the SOFCs. Crystallographic studies of oxygen annealed LaMnO 3 doped mainly with SrO which were reported in the literature revealed the occurrence of a phase transformation starting with orthorhombic structure in the undoped material with the increase of SrO mole fraction. Beyond mole fraction 0.16 the structure was reported to be hexagonal and for a mole fraction 0.20 of SrO the structure was stated to be hexagonal–rhombohedral. However the variation in the room temperature crystal structure with SrO content of ] LSM is not reflected in the DGSrO or even in the plotting of log gSrO against composition at two isothermal temperatures 1000 and 1273 K. Perhaps EMF measurements at closer intervals of mole fraction of SrO might be required to discern phase changes if any with change in SrO content of LSM. The value of a SrO also computed at 1073 K has been found to be 0.054, which indicates its compatibility with La 0.8 Sr 0.2 Ga 12x Mg x O 32y (LSGM) which is contemplated as new electrolyte on SOFC operating at 1073 K [17]. Fig. 3. Temperature dependence of EMF of cells II–IV. Table 3 Least squares expressions for EMF of the galvanic cells I –IV Cell I II a II b III IV EmV 5 a 1 bT (K) a b 51.66 226.12 226.05 234.21 221.55 0.01011 0.07270 0.07306 0.10438 0.04533 T range (K) Precision (6mV) 720.4 745.0 798.0 744.3 736.5 0.13 0.25 0.53 1.22 0.13 –1048.3 –989.5 –997.0 –986.0 –1004.5 SrO dissolved in LaMnO 3 . As a check for the internal consistency between different cell configurations, the cell with a configuration of V was used (Table 4). The values of 45.0 and 50.4 mV recorded at two constant temperatures 897.8 and 986.6 K, respectively, for cell V were found to be in a fair agreement with 39.4 and 46.6 computed from EIII 1 EIV 2 EII by interpolating the corresponding least 4. Conclusion Phase pure strontia doped LaMnO 3 could be synthesized at as lower temperature as 1120 K by resorting to a judicial combination of polymeric gel method with citrate auto combustion techniques. This technique was shown to be Table 4 Test for internal consistency Cell V II III IV Configuration 0.2LSM uu0.1LSM 0.15LSM uu0.065LSM 0.2 LSM uu0.065LSM 0.15LSM uu0.10LSM III1 IV2I I EMF (mV) at 897.8 986.6 K 45.0 39.2 59.5 19.1 39.4 50.4 45.6 69.0 23.2 46.6 Precision 2s (6mV) Remarks 1.1 0.6 2.6 0.4 3.4 Directly measured from V Calculated from EII (Table 3) Calculated from EIII (Table 3) Calculated from EIV (Table 3) EIII 1EIV 2EII S. Arul Antony et al. / Journal of Alloys and Compounds 322 (2001) 113 – 119 Table 5 SrO potentials derived from results of EMF measurements on cells I–IV ] [SrO] LSM DGSrO (kJ mol 21 )5 A 1 BT (K) 0.20 0.15 0.10 0.065 A B a9 b9 211.42 29.84 25.68 24.81 20.01360 20.01950 20.02818 20.03360 20.7105 21.0188 21.4720 21.7550 2596.4 2514.1 2296.8 2251.5 Table 6 Variation of activity co-efficient of SrO [gSrO ] with its mole fraction in LSM [SrO] LSM 0.20 0.15 0.10 0.065 log a SrO 5a91b9 /T (K) log gSrO 5 a0 1 b0 /T (K) log gSrO at a0 b0 1000 K 1273 K 20.0115 20.1949 20.4720 20.5680 2596.4 2514.1 2296.8 2251.5 20.6079 20.7090 20.7688 20.8195 20.4800 20.5987 20.7052 20.7655 superior in terms of lower temperature of synthesis and phase purity to the conventional ceramic route. Since the LaMnO 3 mainly doped with 20 mol.% SrO is the widely accepted cathode material for SOFCs operating in air, the solid fluoride electrolyte EMF method is the only technique that could be used for reliable SrO activity measurements in such cathode materials. A negative deviation from ideality with SrO substitution decreases with increase in SrO content in the four compositions (namely 0.065, 0.10, 0.15 and 0.20 mole fractions of SrO) that were Fig. 4. Variation of logarithm of activity co-efficients with SrO dopant concentration. 119 log a SrO at 1000 K 21.3069 21.5329 21.7688 22.0065 studied over the range approximately 750–1000 K. The negative deviation from ideality in 0.2 LSM insures its compatibility with respect to SrY 2 O 4 formation by reaction with yttria stabilized zirconia electrolyte. Acknowledgements The first author is grateful to UGC Teacher fellowship awarded under FIP (TFNMD-176 IX Plan). The authors are grateful to Rev.Fr. John Prakasam, Director, LIFE, Loyola College and to Dr. Baldev Raj, Director, MCRG, IGCAR and Dr. V.S. Raghunathan, Associate Director, MCG of IGCAR for their keen interest and constant encouragement throughout the course of this work. References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (3) (1993) 563. [2] O.M. Sreedharan, F. Lawrence, T. Mathews, in: S. Selladurai et al. (Eds.), Proceedings of the First Asian Conference on Solid State Ionic Devices, Science and Technology FASSID-2000, 22–24 March, 2000, p. 130. [3] T. Nakamura, G. Petzow, L.J. Gauckler, Mat. Res. Bull. 14 (1979) 649. [4] O.M. Sreedharan, R. Pankajavalli, J.B. Gnanamoorthy, High. Temp. Sci. 16 (1983) 251. [5] N. Kamegashira, Y. Miyazaki, Y. Hiyoshi, Mat. Lett. 2 (1984) 194. [6] R. Hildrum, M. Brustad, W. Changzhen, O. Johannesan, Mat. Res. Bull. 29 (1994) 851. [7] J.H. Kuo, H.U. Anderson, D.M. Sparlin, J. Solid State Chem. 83 (1989) 52. [8] J.A.M. Van Roosmalen, E.H.P. Cordfunke, R.B. Helmholdt, J. 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