I see in publications that people don't directly use strontium oxide to react with Alumina and instead they use other derivatives of Strontium such as Strontium carbonate to form strontium hexaaluminate. I am not sure what the reason is. Can anyone advise please?
Strontium oxide is far too stable. You would have to remove an atom of oxygen from the strontium or the aluminum in order to form the target compound. This would require much, much more energy than the decomposition of strontium carbonate.
The main reason is what Nita mentioned. When working in air you simply cannot precisely weigh the necessary amount of SrO. The same problem exists for all alkali and alkaline earth metals. Oxides of the rare earth metals also easily interact with air yielding hydroxides, but they can be weighed correctly after annealing.
For Al the situation is opposite: Al2O3 has constant composition when exposed to air, so people use it even though it does not provide fast reaction rates.
Prof Dragoe and Alexander have provided the partial reasons for using SrCO3 instead of SrO (which is has large tendency to form hydroxides and carbonates in ambient conditions). We ACTUALLY want SrO to be reactive, and the (amorphous) SrO formed in situ after the decomposition of SrCO3 is even more reactive. But then it gets Al2O3 to react with (assuming the powders are thoroughly mixed) during the heat treatment process, and then Sr easily diffuses into the crystal structure of Al2O3 to form beta-alumina structure. Most of the hexaaluminates have beta-alumina structure (google "beta alumina structure" for more details), only some of them have magnetoplumbite structure (e.g. hexaferrites).
From my experience of synthesizing Ba hexaalluminates by chemical (solution) methods, i always used to get a layer of crystalline BaCO3 formation during solution mixing and ageing. To avoid that I used to purge N2 gas through the reaction mixture to avoid the formation of any Ba carbonate by the interaction with air. This will also bring down the hexaaluminate formation (crystallisation) temperature. Whereas, if carbonates of Ba or Sr are used, the crystallisation of hexaaluminate phase takes place 1100degC and the particles formed will be coarse grained having low surface area, rather than nanocrystallites (if you are interested in catalytic properties, this may not be desirable).
Try using Sr acetate instead of carbonate, you might get better results.
Hope this helps.
Cheers.
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