Synthesis, structure, electrical properties and Na+ migration pathways of Na2CoP1.5As0.5O7

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.