Antiferromagnetic Mott insulating state in single crystals of the honeycomb lattice material Na2IrO3

We have synthesized single crystals of Na2IrO3 and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction, electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility chi versus temperature T, and heat capacity C versus T measurements. Na2IrO3 crystallizes in the monoclinic C2/c (No. 15) type structure which is made up of Na and NaIr2O6 layers alternately stacked along the c axis. The chi(T) data show Curie-Weiss behavior at high T > 200 K with an effective moment mu(eff)=1.82(1)mu(B) indicating an effective spin S-eff= 1/2 on the Ir4+ moments. A large Weiss temperature theta = -116(3) K indicates substantial antiferromagnetic interactions between these S-eff= 1/2, Ir4+ moments. Anomalies in chi(T) and C(T) data indicate that Na2IrO3 undergoes a transition into a long-range antiferromagnetically ordered state below T-N= 15 K. The magnetic entropy at T-N is only about 20% of what is expected for S-eff= 1/2 moment ordering. The reduced entropy and the large ratio theta/T-N approximate to 8 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na2IrO3. In plane resistivity measurements show insulating behavior. This behavior together with the local-moment magnetism indicates that bulk Na2IrO3 is a Mott insulator.