Superconducting Quintuple-Layer Square-Planar Nickelates—Synthetic Strategies and Challenges (Part II)

Since the discovery of superconductivity in Sr_0.2NdNiO₂, square-planar nickelates have become a prime testbed to study superconductivity in a cuprate-like materials analog. The Ruddlesden-Popper (RP) square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to further tune the electronic properties with dimensionality. Our group recently discovered superconductivity in Nd₆Ni₅O₁₂ [1-2]. The layered nickelates, Nd_n+1Ni_nO_2n+2, and their parent Nd_n+1Ni_nO_3n+1 oxygenated forms, have yet to be synthesized in bulk for n &gt; 3. Here, we use atomically-precise molecular beam epitaxy and subsequent oxygen de-intercalation to stabilize RP square-planar nickelate thin films. We explore competing growth requirements for the epitaxial stabilization of the as-grown Nd_n+1Ni_nO_3n+1 material and the subsequent reduction to Nd_n+1Ni_nO_2n+2. The precursor Nd_n+1Ni_nO_3n+1 films synthesized with more than 1% tensile strain form strain-relieving vertical NdO faults. That said, a high-quality reduced phase can be achieved only with tensile strain in the as-grown state due to the expansion of the in-plane lattice constant upon reduction. We demonstrate the synthesis of n = 3 and n = 5 RP square-planar nickelates on NdGaO3(110), which provides sufficient (~ 1%) tensile strain for reduction while minimizing the formation of vertical NdO faults in the as-grown state. Further synthetic optimization of the square-planar RP nickelates could lead to future investigations of nickelate superconductivity with dimensionality as a tuning knob in addition to chemical doping.<br/>[1] GA Pan et al. Superconductivity in a quintuple-layer square-planar nickelate. arXiv:2109.09726.<br/>[2] GA Pan et al. Superconducting Quintuple-layer Square-planar Nickelate Nd6Ni5O12: Superconducting and Electronic properties (Part I). <i>MRS Bulletin</i> Spring 2022.