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
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 99.8% 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...