New-type phase transition of Li2RuO3 with honeycomb structure

A new-type structural transition has been found in Li2RuO3 with a honeycomb lattice of edge-sharing RuO6 octahedra. With decreasing temperature T, the electrical resistivity exhibits an anomalous increase at T = T-c similar to 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 P2(1)/m at Tc. The most striking fact is that a significant reduction of the bond lengths is found between two of the six Ru-Ru pairs of the hexagon in the low-temperature phase, indicating a new-type phase transition driven by the formation of the molecular orbits of these Ru-Ru pairs.