Two types of charge ordering in bismuth nickelate

Abstract

Among the transition-metal oxides, bismuth nickelate is particular in that, under normal conditions, it crystallizes in a triclinic structure with charge ordering in the Bi sites. Upon increasing pressure, a phase transition takes the material to the GdFeO3-type orthorhombic phase commonly found in perovskite oxides; recent attention to BiNiO3 arises, in part, because this transition can be engineered to be accompanied by a colossal negative thermal expansion. On the other hand, the closely related rare-earth nickelates also show the orthorhombic phase, but as temperature decreases they undergo a metal-insulator transition to a monoclinic phase where charge ordering appears instead in the Ni sites. Since bismuth is similar in valence and size to some of the rare-earth atoms, it has been conjectured that a BiNiO3 phase with charge ordering in the Ni sites might also exist—this would make this perovskite unique in that it can harbor charge ordering in either set of cation species, while the other set contains identical nominal ionic charges. In this paper we show that such a phase indeed corresponds to a special point of the energy surface of BiNiO3 when studied with methods based on density-functional theory. The existence of this phase could explain the somewhat conflicting experimental reports regarding the valence state of the Ni ions in the high-pressure phase of BiNiO3.

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