Abstract
Na 9Co 2O 7 was prepared via the azide/nitrate route. Stoichiometric mixtures of the precursors (Co 3O 4, NaN 3 and NaNO 3) were heated in a special regime up to 450 °C and annealed at this temperature for 100 h in specially designed silver containers. Single crystals have been grown by annealing a Na 2O/Na 9Co 2O 7 mixture (molar ratio 1:2) at 500 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure ( Pca2 1, Z=4, a=9.5477(6) Å, b=9.9040(6) Å, c=10.1919(7) Å, 4103 independent reflections, R 1=5.23% (all data)) there are isolated Co IIIO 4 tetrahedra and Co IIO 3 triangles. Na 9Co 2O 7 is isostructural to Na 9Fe 2O 7, and shows Curie–Weiss behaviour down to ∼25 K. The best fit (for Co II ( S=3/2) and Co III ( S=2)) yields an average value of the magnetic moment (per cobalt) μ=3.23 μ B ( g=2.07) and Θ=−6 K. At 20 K one observes an onset of ferromagnetic order.
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