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

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We have synthesized single crystals of Na₂IrO₃ and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction (PXRD), electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility χversus 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 χ(T) data show Curie-Weiss behavior at high T > 200K with an effective moment μₑff = 1.82(1) μB indicating an effective spin Sₑff = 1/2 on the Ir⁴+ moments. A large Weiss temperature θ= - 116(3)K indicates substantial antiferromagnetic interactions between these Sₑff = 1/2, Ir⁴+ moments. Sharp anomalies in χ(T) and C(T) data indicate that Na₂IrO₃ undergoes a transition into a long-range antiferromagnetically ordered state below Tₙ = 15 K. The magnetic entropy at Tₙ is only about 20% of what is expected for Sₑff = 1/2 moment ordering. The reduced entropy and the small ratio Tₙ/θ≈ 0.13 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na₂IrO₃. In plane resistivity measurements show insulating behavior. This together with the local moment magnetism indicates that bulk Na₂IrO₃ is a Mott insulator.