Twin Boundary and Fivefold Twins in Nickel Oxide

Twinning is a common phenomenon in crystal growth and mechanical deformation. It plays a key role in tuning the mechanical and transport properties of metals. In transition metal oxides, the twinning may give additional changes related to magnetism. Herein, the atomic configuration and the electronic structure of the twin boundaries (TBs) in rock-salt-type NiO are studied, combining aberration-corrected transmission electron microscopy and first-principles calculations. For NiO, the twinning occurs on the (111) plane, with oxygen atoms located at the TB and antiferromagnetic (AFM) coupling between the mirroring Ni atomic planes near the TB. The configuration gives the TB energy of 0.26 J m⁻². Atomic and magnetic structures of fivefold twins in NiO are also investigated. In the structure of fivefold twins with the lowest energy, ferromagnetic coupling is formed in the AFM TBs due to the competing exchange interaction at the core of fivefold twins.