Spontaneous phase segregation of Sr₂NiO₃ and SrNi₂O₃ during SrNiO₃ heteroepitaxy

Recent discovery of superconductivity in Nd_0.8Sr_0.2NiO₂ motivates the synthesis of other nickelates for providing insights into the origin of high-temperature superconductivity. However, the synthesis of stoichiometric R_1−xSr_xNiO₃ thin films over a range of x has proven challenging. Moreover, little is known about the structures and properties of the end member SrNiO₃. Here, we show that spontaneous phase segregation occurs while depositing SrNiO₃ thin films on perovskite oxide substrates by molecular beam epitaxy. Two coexisting oxygen-deficient Ruddlesden-Popper phases, Sr₂NiO₃ and SrNi₂O₃, are formed to balance the stoichiometry and stabilize the energetically preferred Ni²⁺ cation. Our study sheds light on an unusual oxide thin-film nucleation process driven by the instability in perovskite structured SrNiO₃ and the tendency of transition metal cations to form their most stable valence (i.e., Ni²⁺ in this case). The resulting metastable reduced Ruddlesden-Popper structures offer a testbed for further studying emerging phenomena in nickel-based oxides.