Honeycomb lattice iridate on the verge of Mott-collapse.

A new honeycomb lattice iridate (La,Na)IrO 3 (≈LaNaIr 2 O 6 ) is successfully synthesized from the spin-orbit coupled Mott insulator Na 2 IrO 3 by replacing the interlayer Na + ions with La 3+ ions. (La,Na)IrO 3 shows a finite Sommerfeld term in heat capacity and a -ln T dependence of resistivity, indicating a realization of a metallic state driven by a Mott collapse. Furthermore, crystal structure analysis reveals the formation of Ir zig-zag chains with metal-metal bonding, increasing kinetic energy resulting in the Mott collapse. This observation would be due to a Mott collapse induced in a J eff = 1/2 spin-orbit coupling Mott insulator with an Ir honeycomb lattice by topochemical control of the ionic configuration.