  A new-type structural transition has been found in Li2RuO3 with honeycomb
lattice of edge-sharing RuO6-octahedra. With decreasing temperature T, the
electrical resistivity exhibits an anomalous increase at T=Tc~540 K, suggesting
the (metal to insulator)-like transition and the magnetic susceptibility also
shows a sharp decrease. Detailed structure analyses have revealed that the high
temperature space group C2/m changes to P21/m at Tc. The most striking fact is
that a significant reduction of the bond lengths is found between two of six
Ru-Ru pairs of the hexagon in the low temperature phase, indicating a new type
phase transition by the mechanism of the formation of molecular orbits of these
Ru-Ru pairs.
