Synthesis, crystal structure, electrical properties, and sodium transport pathways of the new arsenate Na 4 Co 7 (AsO 4 ) 6

A new sodium cobalt (II) arsenate Na4Co7(AsO4)6 has been synthesized by a solid-state reaction and its crystal structure determined from single crystal X-ray diffraction data. It crystallizes in the monoclinic system, space group C2/m, with a=10.7098(9) Å, b=14.7837(9) Å, c=6.6845(7) Å, and β=105.545(9)°. The structure is described as a three-dimensional framework built up of corner-edge sharing CoO6, CoO4 and AsO4 polyhedra, with interconnecting channels along [100] in which the Na+ cations are located. The densest ceramics with relative density of 94% was obtained by ball milling and optimization of sintering temperature, and its microstructure characterized by scanning electron microscopy. The electrical properties of the ceramics were studied over a temperature interval from 280°C to 560°C using the complex impedance spectroscopy over the range of 13MHz–5Hz. The ionic bulk conductivity value of the sample at 360°C is 2.51 10−5Scm−1 and the measured activation energy is Ea=1eV. The sodium migration pathways in the crystal structure were investigated computationally using the bond valence site energy (BVSE) model and classical molecular dynamics (MD) simulations.