  The synthesis, structure, and elementary magnetic and electronic properties
are reported for layered compounds of the type Na3-xMIr2O6 and Na3-xM2IrO6,
where M is a transition metal from the 3d series (M=Zn, Cu, Ni, Co, Fe and Mn).
The rhombohedral structures, in space group R-3m, were determined by refinement
of neutron and synchrotron powder diffraction data. No clear evidence for long
range 2:1 or 1:2 honeycomb-like M/Ir ordering was found in the neutron powder
diffraction patterns except in the case of M = Zn, thus in general the
compounds are best designated as sodium deficient {\alpha}-NaFeO2-type phases
with formulas Na1-xM1/3Ir2/3O2 or Na1-xM2/3Ir1/3O2. Synchrotron powder
diffraction patterns indicate that several of the compounds likely have
honeycomb in-plane metal-iridium ordering with disordered stacking of the
layers. All the compounds are sodium deficient under our synthetic conditions
and are black and insulating. Weiss constants derived from magnetic
susceptibility measurements indicate that Na0.62Mn0.61Ir0.39O2,
Na0.80Fe2/3Ir1/3O2, Na0.92Ni1/3Ir2/3O2, Na0.86Cu1/3Ir2/3O2, and
Na0.89Zn1/3Ir2/3O2 display dominant antiferromagnetic interactions. For
Na0.90Co1/3Ir2/3O2 the dominant magnetic interactions at low temperature are
ferromagnetic while at high temperatures they are antiferromagnetic; there is
also a change in the effective moment. Low temperature specific heat
measurements (to 2 K) on Na0.92Ni1/3Ir2/3O2 indicate the presence of a broad
magnetic ordering transition. X-ray absorption spectroscopy shows that iridium
is at or close to the 4+ oxidation state in all compounds. 23Na nuclear
magnetic resonance measurements comparing Na2IrO3 to Na0.92Ni1/3Ir2/3O2 and
Na0.89Zn1/3Ir2/3O2 provide strong indications that the electron spins are
short-range ordered in the latter two materials. All of the compounds are spin
glasses.
