  We have synthesized the novel material Na_2IrO_3 and studied its structure,
transport, magnetic, and thermal properties using powder x-ray diffraction
(PXRD), electrical resistivity, isothermal magnetization M versus magnetic
field H, static \chi and dynamic \chi_ac magnetic susceptibility versus
temperature T, and heat capacity C versus T measurements. Na_2IrO_3
crystallizes in the monoclinic C2/c (No. 15) structure which is made up of Na
and NaIr_2O_6 layers alternately stacked along the c axis. From a Rietveld
refinement of the PXRD pattern we identify atomic disorder arising from a
mixing of Ir and Na sites within the NaIr_2O_6 layers. The \chi data in H = 1 T
shows Curie-Weiss behavior at high T > 100 K with an effective moment \mu_eff =
1.82(1) \mu_B indicating an effective spin S_eff = 1/2 on the Ir^{4+} moments.
A Weiss temperature \theta = - 62(1) K indicates substantial antiferromagnetic
interactions between these S_eff = 1/2, Ir^{4+} moments. The \chi data in low
field show a sharp cusp at T_g = 5.5 K and there is a bifurcation between
zero-field-cooled (ZFC) and field-cooled (FC) data below this T. The \chi_ac
data also show a sharp cusp at T_g = 5.5 K at a frequency f = 1 Hz which moves
to higher temperatures with increasing f. We did not observe any anomaly at T_g
in our C measurements and only a broad shoulder was observed at a much higher T
= 12 K. Our results indicate that in Na_2IrO_3, a spin-glass like state occurs
below the freezing temperature T_g = 5.5 K and this freezing most likely arises
either from structural disorder or geometrical magnetic frustration.
