Electronic-structure evolution of SrFeO3-x during topotactic phase transformation

Oxygen-vacancy-induced topotactic phase transformation between the ABO2.5 brownmillerite structure and the ABO3 perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO3-x thin films during the topotactic phase transformation. The results show that the increase in oxygen content produces a new unoccupied state of O 2p character near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to the dx2-y2 orbital than to the d3z2-r2 orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO3-x films during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.

Overlooked cation-ordered brownmillerite in Ba2(In1-xGax)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.