Spontaneous Phase Segregation of Sr₂NiO₃ and SrNi₂O₃ during SrNiO₃ Heteroepitaxy

The rich phase diagrams of complex oxide nickelates (RNiO₃) are of critical importance for both materials physics and oxide electronics. Hole doping in RNiO₃ has been studied as an effective strategy for tuning their functional properties, including enhanced electrocatalytic properties found in La_1-xSr_xNiO₃ and superconducting behavior observed in reduced Nd_1-xSr_xNiO₂. However, the synthesis of stochiometric R_1-xSr_xNiO₃ thin films over a range of x has proven challenging, presumably because of the instability of Ni oxidation states greater than +3. 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 co-existing 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 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 the emerging phenomena in nickel-based oxides.