Antiferromagnetism, charge ordering and stretched Ni-O bond in Ln₄Ni₃O₈ (Ln = La, Nd)

Superconductivity was recently observed in Sr-doped NdNiO₂ film after a long pursuit, which inspires us to investigate another Ruddlesden-Popper-based nickelate Nd₄Ni₃O₈ which may hold an antiferromagnetic order and a charge stripe order. Through ab initio calculations, we find that the obtained results turn out to be similar to those of La₄Ni₃O₈. However, we propose that Ni¹⁺ ions in the charge stripe order observed in La₄Ni₃O₈ are in fact antiferromagnetically coupled through a twofold double-exchange mediated by the intermediate Ni²⁺ ion and the stretched Ni¹⁺-O bond. Under high pressure, the extension of the stretched Ni¹⁺-O bond is not favored and the system will be pushed into a meta-stable insulating state. Our picture can successfully explain the temperature dependence of resistivity under high pressure of La₄Ni₃O₈, and shows also consistency with the insulating behavior of Nd₄Ni₃O₈ observed in recent experiment. Considering a +1.33 average valence of Ni in Nd₄Ni₃O₈, which is very close to that of the Sr-doped NdNiO₂, our results support the earlier proposal that a possible way leading to metallicity and even superconductivity is to suppress the existing antiferromagnetism and charge ordering.