Mixed oxides of sodium, antimony (5+) and divalent metals (Ni, Co, Zn or Mg)

A family of α-NaFeO2-type oxides NaxM(1+x)/3Sb(2−x)/3O2 (M=Ni, Co, Zn, Mg; x≈0.8 or 0.9) has been prepared by solid state reactions and characterized by powder XRD. At x=1, ordering occurs with tripling the unit cells and formula units. The powder patterns for Na3M2SbO6 (M=Ni, Co) comply with both trigonal P3112 cell and monoclinic C2/m cell. The Ni compound exhibits also a series of extremely weak reflections (I<0.3%) that need doubling of the c axis, and the final cell is C2/c, a=5.3048(3), b=9.1879(4), c=10.8356(7), β=99.390(5). These ambiguities are explained by stacking faults. The compounds absorb atmospheric moisture with c-axis expansion up to 29%. A delafossite-related superlattice Ag3Co2SbO6 has been prepared by ion exchange and refined: P3112, a=5.3842(2), c=18.6613(10). Ionic conductivity of the Na0.8Ni0.6Sb0.4O2 ceramics, 0.4 S/m at 300 °C, is greater than reported previously, presumably owing to the grain orientation produced by hot pressing.