Crystal chemistry and ion conductivity of the Na1 + xTi2 − xAlx(PO4)3 (0 ≤ x ≤ 0.9) NASICON series

Abstract

The Na_1 + xTi_2 − xAl_x(PO₄)₃ (0.0 ≤ x ≤ 0.9) solid solution has been prepared as a polycrystalline powder. These compounds crystallise in the NASICON type structure, Rc space group, and the crystal structures have been characterised by the Rietveld method with laboratory powder diffraction data. The Ti/Al atoms are randomly distributed over the octahedral site of the NASICON framework. The negatively charged framework is neutralised by the Na⁺ cations which are distributed over the M1 site, fully occupied, and M2 site, partly occupied. The cell parameter evolution along the series agrees with the substitution of larger Ti⁴⁺ by smaller Al³⁺ cations. The thermal expansion coefficients were determined from a thermodiffractometric study of Na_1.8Ti_1.2Al_0.8(PO₄)₃. The ion conductivity of the samples rises as the charge carrier number (x) increases. The activation energies deduced from the Arrhenius plots are close to 0.50 eV. The overall conductivities are near to 10⁻⁵ S cm⁻¹ at 200 °C.