Charge ordering in Ni1+/Ni2+ nickelates: La4Ni3 O8 and La3Ni2 O6

Ab initio calculations allow us to establish a close connection between the Ruddlesden-Popper layered nickelates and cuprates not only in terms of filling of d levels (close to d9) but also because they show Ni1+(S=1/2)/Ni2+(S=0) stripe ordering. The insulating charge-ordered ground state is obtained from a combination of structural distortions and magnetic order. The Ni2+ ions are in a low-spin configuration (S=0) yielding an antiferromagnetic arrangement of Ni1+ S=1/2 ions like the long-sought spin-1/2 antiferromagnetic insulator analog of the cuprate parent materials. The analogy extends further with the main contribution to the bands near the Fermi energy coming from hybridized Ni dx2-y2 and O p states.