Studies on Polymorphic Sequence during the Formation of the 1:1 Ordered Perovskite-Type BaCa0.335M0.165Nb0.5O3-δ (M = Mn, Fe, Co) Using in Situ and ex Situ Powder X-ray Diffraction

Here, we report a synthetic strategy to control the B-site ordering of the transition metal-doped perovskite-type oxides with the nominal formula of BaCa0.335M0.165Nb0.5O3-delta (M = Mn, Fe, Co). Variable temperature (in situ) and ex situ powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), scanning/transmission electron microscopy (SEM/TEM), and thermogravimetic analysis (TGA) were used to understand the B-site ordering as a function of temperature. The present study shows that BaCa0.335M0.165Nb0.5O3-delta crystallizes in the B-site disordered primitive perovskite (space group s.g. Pm (3) over barm) at 900 degrees C in air, which can be converted into the B-site 1:2 ordered perovskite (s.g. P (3) over bar m1) at 1200 degrees C and the B-site 1:1 ordered perovskite phase (s.g. Fm (3) over barm) at 1300 degrees C. However, the reverse reaction is not feasible when the temperature is reduced. FTIR revealed that no carbonate species were present in all three polymorphs. The chemical stability of the investigated perovskites in CO2 and H-2 highly depends on the B-site cation ordering. For example, TGA confirmed that the B-site disordered primitive perovskite phase is more readily reduced in dry and wet 10% H-2/90% N-2 and is less stable in pure CO2 at elevated temperature, compared to the B-site 1:1 ordered perovskite-type phase of the same nominal composition.