Local structure ofLiNiO2studied by neutron diffraction

Abstract

The nature of the magnetic state of $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ 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 ${\mathrm{Ni}}^{3+}$ 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 ${\mathrm{Ni}}^{3+}$ 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 $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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.