Unusual charge distribution in Na_7(CoO_3)_2

摘要

Na_7(CoO_3)_2 was prepared via the azideitrate route. Stoichiometric mixtures of the precursors (Co_3O_4, NaN_3 and NaNO_3) were heated in a special regime up to 450 deg 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 deg 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 (C2/c, Z = 4, a = 14.603(1) A, b = 5.813(1) A, c = 10.586(1) A, beta = 104.968(2) deg, 1841 independent reflections, R_1 = 3.51 percent (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 approx 75 K. The best fit of the susceptibility data yields an average value of the magnetic moment (per cobalt) mu = 3.26mu B (g = 2.09) and theta = -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).