Limits to the strain engineering of layered square-planar nickelate thin films

Abstract

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Amalgams as Hydrogen-Free Reducing Agents for Topotactic Oxide Deintercalation

“…The CaH 2 reduction was indirectly in contact via Al foil, as described previously, and showed a strong film peak indicating successful oxide deintercalation. The reduction using Na 0.25 AlGa shows poor crystallinity, but subsequent reoxidation by annealing under flowing oxygen indicates that this is a reversible process and can be fine-tuned for better crystallinity in the future.…”

“…After heating, the nickelate can be easily separated from the amalgam by mechanically brushing off any amalgam beads from the pellet. Calcium hydride (Alfa Aesar, 92%) reductions, on the other hand, were performed with indirect contact using Al foil as previously described . The furnace was heated to a given temperature at 100 °C/h.…”

“…Unlike the elegant hierarchical synthesis steps common in organic chemistry, solid-state techniques, like high-temperature “shake and bake” methods, hydrothermal synthesis, and flux methods, often lack a priori mechanistic insight, leaving much of synthesis as a blind trial and error process. In more recent years, lower temperature “chimie douce” or soft chemistry topotactic techniques have gained considerable attention as a method to synthesize metastable materials by design through a directed modification process. , In topotactic chemistry, a precursor material’s lattice is treated like a scaffolding to allow for the addition, removal, or rearrangement of targeted atoms. − In these cases, temperatures are not high enough to destabilize the framework of the chemical structure but instead allow for the diffusion of atoms through a lattice to realize kinetically stabilized products. This kinetically stabilized process can therefore enable a mechanistic approach for solid-state synthesis for the rational design of the desired products.…”

Signature of Superconductivity in Pressurized La4Ni3O10

“…Chemical doping or the strain effects from thin films and heterostructures would be effective tools. [9,58] If all goes well, a superconducting La4Ni3O10 phase with flat NiO2 planes may be obtained at lower or even ambient pressure. Related experiments are highly desired and are actually underway.…”

“…[38] After this work, several experimental and theoretical works have been carried out to investigate the underlying mechanisms of high-𝑇c superconductivity in La3Ni2O7, [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] and zero resistance was realized in both single crystal and polycrystalline samples under improved hydrostatic pressure environment. [39,40,43] Among RP series compounds, the trilayer nickelates 𝑅4Ni3O10 (𝑅 = La, Pr, Nd) have attracted more attention since they have been reported to undergo metal-to-metal transitions [28][29][30][31][32][56][57][58][59][60] and the transition was ascribed to intertwined charge and spin density waves. [61] High-𝑇c superconductors, such as cuprates and iron-based superconductors, can always form a broader family of materials with different crystal structures and 𝑇c values.…”