Overlooked cation-ordered brownmillerite in Ba₂(In_1-xGa_x)₂O₅

As Goodenough showed 20 years ago, [1] Ba2In2O5 undergoes a transition from brownmillerite to cubic perovskite at higher temperatures, where it becomes a good oxygen ionic conductor. Past studies on doping gallium onto the indium sites [2] showed that higher Ga contents decrease the temperature required for the disordering to occur, in accordance with a higher structural disorder on the B site. In this work, we have showed that past studies on Ba2(In1-xGax)2O5 have overlooked a special ordering for the Ba2InGaO5 composition. Combined structure refinements of neutron and X ray powder diffraction show that this material adopts a brownmilleite structure below 1200°C with a layered ordering of In and Ga. This is one of a few examples of such an ordering in the brownmillerite structure only due to size difference between the B cations. This ordering further stabilizes the brownmillerite crystal structure compared to other compositions at smaller Ga contents. As could be expected, such stronger ordering is detrimental to ionic conductivity. At 1300°C, the material disorders into a cubic perovskite structure, which was already observed in past studies. However this polymorph has an unreported red colour, which was unexpected for a material with no incompletely filled orbitals. Incorporation of oxygen interstitial defects, on the basis of conductivity versus variable oxygen experiments and spectroscopic measurements, could be at the origin of this colouration much like in TiO2. [3] Unfortunately photocatalytic activity for water splitting could not be tested due to the strong instability of this material towards water.