  The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+
cations on cooling below 270C, where the magnetic susceptibility vanishes. We
use density functional theory calculations to show that this reflects the
formation of a 'valence bond crystal', with a strong bond disproportionation.
On warming, x-ray diffraction shows that discrete three-fold symmetry is
regained on average, and the dimerization apparently disappears. In contrast,
local structural measurements using high-energy x-rays, show that disordered
dimers survive at the nanoscale up to at least 650C. The high temperature phase
of Li2RuO3 is thus an example of a valence bond liquid, where thermal
fluctuations drive resonance between different dimer coverages, a classic
analogue of the resonating valence bond state often discussed in connection
with high T$_c$ cuprates.
