Local structure of LiNiO ₂ studied by neutron diffraction

The nature of the magnetic state of LiNiO ₂ has been controversial. In this compound Ni spins (S = 1/2) form a triangular lattice with the possibility of magnetic frustration, but the exact state of spin correlation has not yet been known in spite of the extensive research work. A factor that complicates understanding of the magnetic state is the orbital state of Ni ³⁺ which is a Jahn-Teller (JT) ion. While there is no signature of long-range Jahn-Teller distortion, local JT distortion has been suspected. We have performed neutron diffraction and atomic pair-density function analyses up to unprecedented large distances to discover a number of unusual features, such as anomalous peak broadening, local JT distortion, sharp oxygen-oxygen distance correlations, and inverted temperature dependence of medium range correlation. These observations are best explained by local orbital ordering of Ni ³⁺ ions into three sublattices. This orbital ordering, however, cannot develop into long-range order because of the strain field it generates, and domains of about 10 nm in size are formed. Domains are susceptible to random pinning by impurities (site disorder) resulting in the loss of structural long-range order. We suggest that this local orbital ordering is the basis for the complex magnetic properties observed in this compound.