Structure and properties of Na_xM₂SbO₆ · yH₂O, M = Co(III), Ni(III) honeycomb oxyhydrates

The oxyhydrates Na_0.85Co₂SbO₆ · 1.7H₂O and Na_0.95Ni₂SbO₆ · 1.5H₂O were prepared by Br₂ oxidation of Na ₃Co₂SbO₆ and Na₃Ni ₂SbO₆ and exposure to ambient air. Their composition and structure are characterized by thermogravimetric analysis, elemental analysis and powder X-ray diffraction. Both form monolayer hydrates with metal cations (Co or Ni) near the 3+ oxidation state. In the parent materials, MO₆ octahedra surround SbO₆ units, forming a honeycomb geometry; Rietveld refinement of the hydrate phases indicate that this motif is retained. Magnetic susceptibility plots display paramagnetic behavior and Curie-Weiss fits support the 3+ oxidation state (low spin d⁶ s=0 for Co and low spin d ⁷ s = 1/2 for Ni). Close inspection of the Ni hydrate's susceptibility reveals a feature at 6.2 K and a nonlinear magnetization at 2 K with no saturation up to 9 T. Heat capacity measurements show a large amount of excess entropy in the Ni hydrate in the 2-20 K temperature range, while the heat capacity of the Co hydrate can be described by a simple lattice contribution. The Ni hydrate sample is easily dehydrated by heating above 100 °C and forms a new Na-deficient phase, Na_0.95Ni₂SbO₆, which is stable at room temperature. Magnetic susceptibility of this phase is consistent with the Ni atom remaining in the 3+ oxidation state after dehydration.