Synthesis, electrical properties and Na+ migration pathways of Na2CuP1.5As0.5O7

Abstract

A new member of sodium metal diphosphate-diarsenate, Na2CuP1.5As0.5O7, was synthesized as polycrystalline powder by a solid-state route. X-ray diffraction followed by Rietveld refinement show that the studied material, isostructural with β-Na2CuP2O7, crystallizes in the monoclinic system of the C2/c space group with the unit cell parameters a = 14.798(2) A; b = 5.729(3) A; c = 8.075(2) A; β = 115.00(3)°. The structure of the studied material is formed by Cu2P4O15 groups connected via oxygen atoms that results in infinite chains, wavy saw-toothed along the [001] direction, with Na+ ions located in the inter-chain space. Thermal study using DSC analysis shows that the studied material is stable up to the melting point at 688 °C. The electrical investigation, using impedance spectroscopy in the 260-380 °C temperature range, shows that the Na2CuP1.5As0.5O7 compound is a fast-ion conductor with δ350°C = 2.28 10-5 Scm-1 and Ea = 0.6 eV. Na+ ions pathways simulation using bond-valence site energy (BVSE) supports the fast three-dimensional mobility of the sodium cations in the inter-chain space.