Abstract
Sodium cobalt di(phosphate/arsenate) Na2CoP1.5As0.5O7 has been synthesized as polycrystalline powder and single crystals by solid state reaction. The structure has been determined by X-ray diffraction at room temperature. The title material crystallizes in the tetragonal system, space group P42/mnm with the unit cell parameters: a = 7.764(3) Å, c = 10.385(3) Å, V = 626.0(4) Å3 and Z = 4. The two tools of crystal structure validation, Bond Valence Sum (BVS) and Charge distribution (CHARDI) methods, have confirmed the crystal structure model. The anionic framework is built of layers of corner sharing (P/As)O4 and CoO6 polyhedra. The sodium atoms are located in the interlayer space. Quantitative analysis using ICP-MS is used to confirm the elemental composition of the polycrystalline powder. Thermal analyzes show a phase transition at 675 °C before the melting point of 700 °C. The electrical properties of the title compound have been characterized by impedance spectroscopy in the 240°C–360 °C temperature range. At 240 °C, the conductivity value of the sample with relative density of 85% is 4 10−6 Scm−1 and the activation energy was Ea = 0.56 eV. The calculated conductivity corrected for porosity is σd (240°C) = 1.88 10−5 Scm−1. The Na+ transport pathways in the interlayer space was simulated using the Band Valence Site Energy (BVSE) model. The BVSE model was also used to explain the effect of the P/As substitution on the electrical properties of the title compound.
Correlation between structure and ionic conductivity using BVSE model simulation. [Display omitted]
•Na2CoP1.5As0.5O7 was synthesized by solid state reaction and its structure determined by XRD.•DTA-TG analysis were used to determine the thermal stability of Na2CoP1.5As0.5O7 of the tetragonal form.•Electrical properties of Na2CoP1.5As0.5O7 was characterized by impedance spectroscopy in the 240°C–360 °C temperature range.•Na+ migration pathways are simulated by means of the bond valence site energy (BVSE) model.