Spontaneous phase segregation of SrNiO and SrNiO during SrNiO heteroepitaxy.

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