  We report results of our study of a newly synthesized honeycomb iridate
NaxIrO3 (0.60 < x < 0.80). Single-crystal NaxIrO3 adopts a honeycomb lattice
noticeably without distortions and stacking disorder inherently existent in its
sister compound Na2IrO3. The oxidation state of the Ir ion is a mixed valence
state resulting from a majority Ir5+(5d4) ion and a minority Ir6+(5d3) ion.
NaxIrO3 is a Mott insulator likely with a predominant pseudospin = 1 state. It
exhibits an effective moment of 1.1 Bohr Magneton/Ir and a Curie-Weiss
temperature of -19 K but with no discernable long-range order above 1 K. The
physical behavior below 1 K features two prominent anomalies at Th = 0.9 K and
Tl = 0.12 K in both the heat capacity and AC magnetic susceptibility.
Intermediate between Th and Tl lies a pronounced temperature linearity of the
heat capacity with a large slope of 77 mJ/mole K2, a feature expected for
highly correlated metals but not at all for insulators. These results along
with comparison drawn with the honeycomb lattices Na2IrO3 and (Na0.2Li0.8)2IrO3
point to an exotic ground state in a proximity to a possible Kitaev spin
liquid.
