Crystal structure and ionic conductivity of the new cobalt polyphosphate NaCo(PO3)3

• A new phosphate NaCo(PO3)3 was obtained by solid state reaction.
• The Structure was determined from powder X-ray diffraction.
• The CIS measurements were optimized and the obtained spectra were fitted by electrical equivalent circuits.
• The conduction pathways for the Na+ cations are simulated by means of the bond valence site energy model.

Polycrystalline sample of sodium cobalt triphosphate NaCo(PO3)3 was obtained by solid-state reaction and characterized by X-ray powder diffraction. The title compound is isostructural to NaZn(PO3)3 and its structure was refined by the Rietveld refinement in the cubic system, space group Pa  3̅, with a=14.2484(4) Å. The obtained structural model is supported by bond valence sum (BVS) and charge distribution (CD) methods. The structure is described as a three-dimensional open-anionic framework built up of corner-sharing CoO6 and PO4 polyhedra with interconnecting channels along the 3-axis, in which the Na+ cations are located. The ionic conductivity measurements are performed on pellets with relative density of 84%. The electrical conductivity is 1.01×10−5 S cm−1 at 480 °C and the activation energy deduced from the slope is 1.1 eV. The correlation between crystal structure and ionic conductivity was studied by the means of the bond-valence-site-energy (BVSE) model. The main result is that sodium transport is made mainly via Na2 and Na3 sites.

Rietveld refinement patterns of the NaCo(PO3)3 sample and projection of the structure of along [111] directions.Figure optionsDownload as PowerPoint slide