Unusual charge distribution in Na 7(CoO 3) 2

Abstract

Na 7(CoO 3) 2 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 the subsequent annealing of the reaction product at 500 °C for 1000 h, in silver crucibles which were sealed in glass ampules under dried Ar. According to the X-ray analysis of the crystal structure at 273 K ( C 2 / c , Z = 4 , a = 14.603 ( 1 ) Å , b = 5.813 ( 1 ) Å , c = 10.586 ( 1 ) Å , β = 104.968 ( 2 ) ° , 1841 independent reflections, R 1 = 3.51 % (all data)) there is only one independent position of Co atoms, which are trigonally planar coordinated by oxygen. The CoO 3 triangles with cobalt in an average oxidation state of +2.5 are isolated from each other. No charge ordering was also detectable by X-ray single crystal diffraction at 100 K. Na 7(CoO 3) 2 shows Curie–Weiss behaviour down to ∼ 75 K . The best fit of the susceptibility data yields an average value of the magnetic moment (per cobalt) μ = 3.26 μ B ( g = 2.09 ) and Θ = − 42 K , supporting strongly the presence of Co +2 ( S = 3 / 2 ) and Co +3 ( S = 2 ) , in equal amounts. At 11 K one observes an onset of antiferromagnetic order. The substance is semiconducting with a band gap of 47.7 kJ (0.495 eV).