Emergent Behavior in Ni¹⁺-Rich Layered Nickelates

For more than 30 years nickel oxides have been explored in vain as potential cuprate-like superconductors. The recent report of superconductivity in a nickelate film by Hwang et al (Nature 572, 624 (2019)) brings new urgency to this quest. Unlike that of their cuprate analogs, the electronic phase diagram of layered nickelates, R_2-xSr_xNiO₄ (R=rare earth) is populated not by metals and superconductors, but rather with insulating charge- and spin-stripe phases. The absence of superconductivity can be rationalized by factors such as d-p mixing, lack or planar orbital polarization, among others. Each of these traces ultimately to the markedly different electronic configuration of d⁸ Ni²⁺ in octahedral coordination vis-à-vis d⁹ Cu²⁺ in an essentially square planar environment. Ni¹⁺-containing solids are rare and difficult to synthesize. However, a family of such materials with square planar Ni does exist, with formula R_n+1Ni_nO_2n+2, where the Ni¹⁺ fraction is given by (n-1)/n. Indeed, the reported nickelate superconductor thin film derives from the Sr-doped ‘infinite layer’ endmember of this series, Nd_0.8Sr_0.2NiO₂. Here we discuss the phase behavior of the n=3 member of this series, R₄Ni₃O₈(R=La,Pr) made possible by advances in high pressure crystal growth. Specifically, we show the emergence in this system of several characteristics of cuprates that have been missing in nickelates, including strong in-plane x²-y² orbital polarization, significant 3d-2p mixing at E_f, and competition between metallic and antiferromagnetic insulating stripe phases. We present an expanded phase diagram of the reduced, layered nickelates and suggest ways in which these materials might be made superconductors.