Strontium al corresponding to anti ferromagnetic ordering. However,
Strontium Manganese Oxide Introduction The ternary inter oxide SrMnO3 was first characterized by XRD by Negas and Roth . This perovskite type inter oxide crystallizes in the hexagonal form with the space group P63/mmC(194). This inter oxide is of considerable interest to materials scientists owing to its ability to accommodate a high degree of anion vacancies, crystal structure and its high Neel Temperature (TN). It also exhibits polymorphism with two different cubic and hexagonal structures. The value of TN was reported to be 350 K by Chamberland et al corresponding to anti ferromagnetic ordering.
However, Battle et al concluded TN to be 278 ± 5 K from Massbauer and neutron diffraction studies on 57Fe doped SrMnO3 which value of TN was expected to be little different from that for undoped one. The variation of electrical conductivity with oxygen stoichiometry and the correlation of high temperature stability with structure were also reported by Battle et al besides others , . The results of this synthesis and XRD characterization as well as measurement of SrO activities in this inter oxide at higher temperatures under 1 atmosphere of oxygen are also presented here.An attempt to thermodynamically characterized the product of hydrogen reduction of SrMnO3 using a 15YSZ based emf method is also described. 2. Experimental 2. 1.
Materials Mn3O4 ( purity better than 99. 99%,Aldrich, USA) and SrCO3 ( better than 99. 9%, IDPL India) were the main starting materials. Reagent grade ethylene glycol ( better than 99. 91%, Aldrich, USA), citric acid (better than 99. 8%, Merck India), and nitric acid ( better than 99%, Fischer, India ) were used for the hybrid polymeric gel cum combustion route.
Better than 99. 99% pure SrF2 (Aldrich) and 15YSZ ( purity better than 99. % and density greaer than 98% theoretical, Yamori, Japan) were used for the preparation of electrolyte as well as for use as an electrochemical catalyst in the electrodes. 2. 2.
Procedure Phase pure SrMnO3 was synthesized by the PGC method. In this hybrid method, required quantities of SrCO3 and MnO (prepared by reducing Mn3O4 in H2 stream at 1173 K for 20 hours) were dissolved in 50 % excess of warm 1:3 HNO3 separately. These solutions were then carefully added to the hot ester which in turn was prepared by heating equi-molar mixture of citric acid and ethylene glycol.The heating was accompanied by constant stirring up to 360 K for 1-2 h. While heating the syrupy liquid, which consisted of a mixture of nitrate and ester in the mole ratio of 10 : 1 with excess of citric acid, the temperature was gradually raised to 620 K and maintained for 3-4 h. The colour of the viscous solution gradually changed from yellow to red followed by setting to a brown coloured but a 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 of 12-mm diameter by applying a pressure of 100 MPa. These pellets were then calcined at 900 K for 1 h and subjected to final sintering at 1120 K for 4 h.
The thermal stability of SrMnO3 thus obtained was studied by a TG run recorded in static air at a heating rate of 10 K/min up to 1370 K. SrMnO2. 5 was prepared by the reduction of compacted pellets of SrMnO3 in a stream of undried H2 at 873 K for 6 h followed by cooling in the same atmosphere to room temperature.For the preparation of recrystallized SrF2 electrolyte, well-dried SrF2 powder was compacted into cylindrical pellets of 15-mm diameter by applying a pressure of 180 MPa.
These pellets were kept over a recrystallized zirconia boat in a Pt wire wounded tubular furnace and slowly heated to 1693 K at the heating rate of 5 K/min under a controlled flow of pure and dry argon and maintained for 2 h allowing recrystallization to take place. Argon was purified by passing through various columns of suitable drying agents.Subsequently, the temperature was reduced to 1523 K at a slow cooling rate of 3 K/min and maintained for 12 h in order to avoid vaporization, which would otherwise lead to porosity. Strontia was prepared by compacting the dry powder into cylindrical pellets as mentioned earlier, were subjected to heat treatment up to 1473 K under the flow of purified oxygen for 5 h, followed by cooling in a steam of high purity helium down to a temperature of 1073 K and quenched in order to avoid the formation of higher oxides.
2. 3. emf measurements 2. 3. 1.
Oxide emf measurements For the oxide electrolyte cells, the test electrodes were made from the equi-molar mixture of SrO + MnO + SrMnO2. 5 by powder compaction. A two compartment cell assembly was employed in which the 15 YSZ was used to separate the inner air/Pt reference electrode from the outer test electrode compartment with purified argon cover gas. 2. 3. 2. Fluoride emf measurements The anode and cathode pellets fro the fluoride EMF measurements were prepared from equi-molar mixtures of SrO + SrF2 and SrMnO3 + SrF2 by powder compaction as already described.
The reference electrode material, namely SrO/SrF2, with traces of platinum black, was prepared from a 5:4 intimate mixture of the two compounds and were immediately compacted into cylindrical pellets without exposing to the ambient atmosphere under a cover of an inert gas. For the measurement of emf, an open-cell-stacked-pellet assembly was used. High purity O2 (purity better than 99. 98% ) was dried by passing through suitable drying agents at a very low flow rate. Other details of the emf measurements were reported elsewhere . Thermocouples used for temperature measurements were calibrated against a standard Pt-10% Rh/Pt thermocouple. .
Results and discussion The product of the polymeric-gel combustion method yielded phase pure SrMnO3, whose XRD pattern is given in Fig. 1. The pattern was indexed and was found to conform to the hexagonal phase. The synthetic method adopted could be considered as a hybrid of the polymeric-gel methods , and combustion methods including citrate combustion earlier demonstrated by Pankajavalli et al . In this hybrid method, gelation process ensures the homogeneous distribution of cations in the gel matrix, thereby precluding segregation of 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 nitrate as an oxidant triggering auto combustion. The geleation process was followed by the colour changes from colourless transparent gel through brown and red corresponding to the dehydration and decarboxylation reactions of excess citric acid , thereby avoiding excessive heating. Sintered pellets of SrF2 intended to be used as solid electrolyte was tested for the absence of SrO by XRD and was found to be phase pure SrF2.The density of this pellet as measured by the physical dimensions was better than 94%. For the galvanic cell I (fluoride emf measurements over the range 753 – 1032 K), Pt, SrO, SrF2, O2 (1atm) ¦ SrF2 ¦O2 (1atm), SrMnO3, SrF2, Pt I Single crystal CaF2 could not be used as the solid electrolyte due to the problem of compatibility with SrO of unit activity present in the reference electrode on account of the reaction SrO (s) + CaF2 (s) > CaO (s) + SrF2 (s) (1) The standard Gibbs energy change, ? GoR per mole of O2 was estimated to be ?GoR (kJ mol -1) = -63. 4 + 0. 0143 T (2) from the tabulated values of ? G compiled by Knacke et al.
after fitting into a two-term expression valid over the temperature range 800-1100 K. At 1000 K, the ? GoR is thus seen to be -49 kJ, which is indicative of the spontaneity of reaction (1)AA. For the oxide emf measurements, the galvanic cell II with the configuration Pt, SrO, MnO, SrMnO3¦ 15YSZ ¦ N2-O2, Pt II was employed over the temperature range of 874-1058 K.The emf results for cells I and II are given in Tables 1 and 2 respectively.
These results plotted in Figures 2 and 3 could be represented by the least squares expression EI ± 1. 65 (mV) = 98. 33 + 0.
0448 T (K) (753-1032K) (3) and EII ± 2. 51 (mV) = 1221. 87 – 0.
4874 T (K) (874-1058 K) (4) respectively. Two runs were carried out with a slight difference in the mole ratios of the test electrode in order to ascertain the equilibrium nature of the emf results as is always done in emf measurements .Such variation in composition to the extent of 10 mol% was resorted to as a test for the equilibrium nature of the emf values because any other reference electrode (such as CaO/CaF2) if used for checking internal consistency would invariably add large uncertainities in the derived Gibbs energy data as already mentioned. The flow rate independent emf of the N2-O2 mixture was separately determined using an oxygen sensor based on 15YSZ electrolyte with air/Pt as the reference electrode (Table 3) E(N2-O2) ± 0. 43 (mV) = -0.
0489 + 0. 8242 T (K) (5) After correcting for the standard state of oxygen in the reference air/Pt electrode, the emf of the N2-O2 mixture was found to conform to the expression E (ref) ± 0. 43 (mV) = -0.
0489 + 0. 11692 T (K) (6) The half cell reactions and the over-ll virtual cell reaction for cell I corresponding to two faraday of electricity are given as follows SrO (s) + 2F- ? SrF2 (s) + ? O2 (g) + 2e- (7a) SrF2 (s) + ? O2 (g) + 2e- ? SrOSrMnO3 + 2F- (7b) And the over-all reaction is therefore SrO (s) ? SrOSrMnO3 (7) The standard Gibbs energy change for reaction (7) is given by ?GoR(7) = -2FEI (8) RT ln aSrO ± 0. 32 (kJ mol-1) = -18. 97 – 0.
00865 T (K) (9) This leads to the expression log aSrO ± 0. 02 = -0. 4518 – 991.
/ T (K) (10) From this expression of log aSrO, log ? SrO could be derived by using ASrO = ? SrO . xSrO where xSrO = 0. 5 as in SrMnO3 and is given as log ? SrO(SrMnO3) = -0. 7528 – 991. 3/T (K) (11) These results along with similar results on 0.
2 SrO substituted Lanthanum manganite LSMj. all are used to evaluate their compatibility with materials in contact with them. The standard Gibbs energy of formation, ? Gof, ox of SrMnO3 from its constituent binary oxides, as per the reactionSrO (s) + MnO2 (s) ? SrMnO3 (s) (12) Is given by the relation ?Gof, ox = aSrMnO3 / aSrO.
a MnO2 = RT ln aSrO + RT ln aMnO2 (13) At this juncture, it should be pointed out that MnO2 should dissociate into Mn2O3 under 1 atm of O2 at 734 K as per the recent Gibbs energy data derived by Fritsch and Navrotsky 11 from careful calorimetric measurements. By fitting their value of RT lnPO2 at 698 and 798 K for the Mn2O3/MnO2 equilibrium, the following expression was arrived atLog PO2 = 11. 33 – 8324 / T (K) (14) From which the temperature of dissociation was derived to be 734 K. Owing to the sluggishness of MnO2 dissociation, this value of 734 K could not be corroborated by TG on MnO2 in 1 atmosphere of pure O2. Since the emf results on cell I could be initiated only from 753 K, it was not possible to study the activity of SrO unequivocally fixing the activity of SrO by configuring the test electrode to be Pt, MnO2, SrMnO3, SrF2, O2 (1 atm).Another limitation in so choosing the electrode was the lack of knowledge of the phase diagram of the system Sr-Mn-O.
Though there are two SrO rich ternary phases in the pseudo binary system SrO-MnO2 with the stoichiometries Sr3Mn2O7 12, and ? -Sr2MnO4 13, There is no interoxide between SrMnO3 and MnO2 along the tie line temperatures of lower than 734 K pure MnO2 cannot co-exist with SrMnO3 without except for the terminal solution of sotichiometry Sr0. 72Mn8O16 14. Therefore, even at extracting SrO. Hence, the approach of measuring aSrO in the single phase SrMnO3 is justified but RT ln aSrO could not be taken as equivalent to ?Gof, ox due to the absence of co-existence of excess MnO2 with SrMnO3. Taking the high dissociation pressure of MnO2 into account, the values of RT ln aSrO could be taken to yield highest limit of stability. Thus, at 973 K, this limiting value of ?Gof, ox could be taken to be -27.
4 kJ ( assuming RT ln aMnO2 to be zero if not positive due to exceeding its temperature limit of stability). The emf expressions (4) and (6) were combined to yield EIII ± 2. 94 ( m V) = 1221. 87 – 0.
37052 T (K) ( 874-1058 K) (15) For the emf of the hypothetical cellPt, SrO, MnO, SrMnO2. 5 ¦ 15YSZ ¦ O2 ( 1atm), Pt III The standard Gibbs energy change, ? GoR for its virtual over-all cell reaction SrO + MnO + ? O¬2 (g) ? SrMnO2. 5 (16) Corresponding to one faraday of electricity was calculated from the expression (14) to be ?GoR ± 0. 28 ( kJ mol-1) = -117. 89 + 0. 03575 T (K) (17) In order to calculate the ? Gof, ox of SrMnO2.
5 from its constituent oxides as per reaction SrO + MnO1. 5 ? SrMnO2. (18) One would require the ? GoR for the conversion of MnO to MnO1. 5. For this purpose, the Gibbs energy data from the oxide emf studies 15- 18 on the buffer mixtures MnO/Mn3O4 and Mn3O4/Mn2O3 were assessed by Sreedharan and Mallika 19 to yield the expression ?GoR 19(kJ mol-1) = -93.
042 + 0. 0524 T (K) (19) corresponding to the reaction MnO + ? O¬2 (g) ? MnO1. 5 (20) Combining the expressions (17) and (19) one obtains Gof, ox(SrMnO2. 5) ( kJ. mol -1) = -24. 85 – 0. 0166 T (K) (21) for the standard Gibbs energy change for the reaction (18).
The relatively small magnitudes of the positive entropy changes +9 and 17 JK-1mol-1 Corresponding to the solid-solid reactions (7) and (18) respectively could be considered as reasonable. This is so due to the fact that the ideal entropy of mixing of two solids in equi-molar ratio is of the order of 6 JK-1mol-1 in the absence of other contributions to the entropy of solution.