Superconductivity realization in La₂NiO_4+δ by tuning the symmetry group by topotactic reduction

The discovery of high-temperature superconductivity in layered Ruddlesden–Popper nickelates (La _n+1Ni_nO3_n+1), particularly in the n = 2 ( La₃Ni₂O₇) and n = 3 (La₄Ni₃O_10+δ) members under high pressure, has renewed the interest in the nickelate family. Under pressure, the n = 2 undergoes a transition from Amam to Fmmm or I4/mmm, while the n = 3 member transforms from P2₁/a to I4/mmm. These structural transitions straighten the Ni–O–Ni bond angle towards 180⁰ and enhance the overlap of Ni 3d_z² orbitals, features expected to contribute to the emergence of superconductivity. In contrast, bulk La₂NiO₄ (n = 1) with the I4/mmm symmetry is reported to be non-superconducting and antiferromagnetic, probably due to the lack of 3d_z² orbital overlap. Tuning its space group could enable superconductivity. For La₂NiO_4+δ, the space group is reported to depend on the oxygen stoichiometry. It was recently observed that the space group can change using CaH₂ and NaH as topotactic reducing agents under various controlled annealing conditions from the I4/mmm to Bmab, Bbcm, and Fmmm. Further experiments are underway to determine what happens to the Ni-O-Ni bond angle as the space group changes. Thus, the n = 1 La₂NiO_4+δ nickelate has the potential to be tuned into a superconducting phase.