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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, and 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 Na0.32Ni1/3Ir2/3O2 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. Na-23 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. Na0.62Mn0.61Ir0.39O2, Na0.80Fe2/3Ir1/3O2,
Na0.90Co1/3Ir2/3O2, Na0.92Ni1/3Ir2/3O2, Na0.86Cu1/3Ir2/3O2 and
Na0.89Zn1/3Ir2/3O2 are spin glasses. (CSD-numbers: Na0.62Mn0,61Ir0.39O2:
426657, Na0.80Fe2/3Ir1/3O2: 426659, Na0.90Co1/3Ir2/3O2: 426658,
Na0.92Ni1/3Ir2/3O2: 426656, Na0.86Cu1/3Ir2/3O2: 426655, and
Na2.8ZnIr2O6: 426660.) (C) 2013 Elsevier Inc. All rights reserved.
