The hidden story in BaNiO₃ to BaNiO₂ transformation: adaptive structural series and NiO exsolution

BaNiO₃ crystal to BaNiO₂ crystal transformation is reported. Contrary to an intuitive topochemical reduction, a two step reaction was observed. In the first step, NiO exsolution occurs and intermediate Ba_1+xNiO₃ phases were obtained and isolated. A composite approach was used to solve the novel structure for x ∼ 1/6 with charge ordered Ni²⁺ and Ni⁴⁺. We argue that this NiO exsolution is responsible for the increased oxygen enhanced reactivity recently reported. Upon re-oxidation, oxygen-deficient mixed valent BaNi^3/4+O_3−x are obtained such that the full redox cycle is irreversible and goes through a diversity of structural and nickel valence adaptative oxides.