LiVO${}_2$ is a model system of a valence bond solid (VBS) in a $3d^2$ triangular lattice. The origin of VBS formation has remained controversial. We investigate the microscopic mechanism by elucidating the $d$ orbital character via on-site ${}^{51}$V NMR measurements in a single crystal up to 550 K across a structural transition temperature $T_c$. The Knight shift, $K$, and nuclear quadrupole frequency, $\delta \nu$, show that the $3d$ orbitals with local trigonal symmetry are reconstructed into a $d_{yz}{d}_{zx}$ orbital order below $T_c$. Together with the NMR spectra with threefold rotational symmetry, we confirm a vanadium trimerization with $d$-$d$ $\sigma$ bonds. The Knight shift extracts the large Van Vleck orbital susceptibility, ${\chi }^{\mathrm{VV}}=3.6\times {10}^{-4}$, in a paramagnetic state above $T_c$, which is comparable to the spin susceptibility. The results suggest that the orbitally induced Peierls transition in the proximity of the frustrated itinerant state is the dominant driving force of the trimerization transition.

• Received 21 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.075135