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

Abstract

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.