Phase Formation and Ionic Conductivity of Na1 + 2xZnxZr2 – x(PO4)3 Phosphates

Na1 + 2xZnxZr2 – x(PO4)3 phosphates have been prepared by a sol–gel process followed by heat treatment. A limited series (0 ≤ x ≤ 0.4) of solid solutions with the NASICON structure has been obtained in the system studied. The crystal structure of Na1.8Zn0.4Zr1.6(PO4)3 has been refined by the Rietveld method. The results demonstrate that the structural basis of this phosphate has the form of a mixed framework made up of corner-sharing (Zr/Zn)O6 octahedra and PO4 tetrahedra. The Na+ ions partially occupy two types of structural voids. The sodium ion conductivity of the Na1 + 2xZnxZr2 – x(PO4)3 phosphates has been studied using impedance spectroscopy. Their conductivity has been shown to increase with increasing carrier concentration, reaching 2.7 × 10–4 S/cm at 723 K in the case of Na1.8Zn0.4Zr1.6(PO4)3. It has been shown that increasing the degree of zinc substitution for zirconium leads to a change in the mechanism of defect formation in the materials studied: from intrinsic disorder at x = 0 to impurity-related disorder for x > 0.2. We have estimated the enthalpy of intrinsic sodium ion disorder (72 kJ/mol) and the activation energy for sodium migration (61 kJ/mol).