HELP-Synthesis Issue

I need to learn how to make Tb oxide. I have metallic Tb that I have been firing @ ~(1000-1100)C for a while to oxidze it. This is working
well, but the question is which oxide(s) I may be forming. References say that TbO2 and Tb2O3 form. So how do I get to one or the other (high quality) so I can use this material for massing other complex oxides? I have a TGA and can synthesize via TGA if necessary, but if you are familiar with this instrument you are aware that this method is a bit time consuming. I can also do powder XRD to check relative sample purity. However, I don't have a reference pattern/paper handy so any help there would save me some time as well.
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