Quantum confinement induced molecular correlated insulating state in La4Ni3O8.

The recently synthesized layered nickelate La4Ni3O8, with its cupratelike NiO2 layers, seemingly requires a Ni1 (d(8))+2Ni2 (d(9)) charge order, together with strong correlation effects, to account for its insulating behavior. Using density functional methods including strong intra-atomic repulsion (Hubbard U), we obtain an insulating state via a new mechanism: without charge order, correlated (Mott) insulating behavior arises based on quantum-coupled, spin-aligned molecular Ni2-Ni1-Ni2 d(z)(2) trimer states across the trilayer (molecular rather than atomic states), with antiferromagnetic ordering within layers. The weak and frustrated magnetic coupling between cells may account for the small spin entropy that is removed at the Néel transition at 105 K and the lack of any diffraction peak at the Néel point.