Synthesis and characterization of equimolar Al/Y-substituted NASICON solid solution Na1+2x+yAlxYxZr2−2xSiyP3−yO12

An equimolar substitution of Zr4+ with Al3+ and Y3+ was chosen to prepare two NASICON series, Na1+2xAlxYxZr2−2x(PO4)3 and Na3+2xAlxYxZr2−2x(SiO4)2(PO4)3, which were compared with previously investigated Sc3+-substituted NASICON materials. The common feature of all series is a mean effective ionic radius of the transition metal cations of 0.718 Å < reff < 0.745 Å. Na1+2xAlxYxZr2−2x(PO4)3 (0 ≤ x ≤ 0.3) has a rhombohedral crystal structure, whereas Na3+2xAlxYxZr2−2x(SiO4)2(PO4)3 changes the modification from a monoclinic (0 ≤ x ≤ 0.1) to a rhombohedral structure (x > 0.1). The average linear coefficient of thermal expansion of Na3+2xAlxYxZr2−2x(SiO4)2(PO4)3, obtained from high temperature X-ray diffraction, increased when x was raised from 4.1 · 10−6 K−1 to 8.1 · 10−6 K−1. The ionic conductivity of Na1+2xAlxYxZr2−2x(PO4)3 compositions was one to two orders of magnitude lower than the Na3+2xAlxYxZr2−2x(SiO4)2(PO4)3 series. For the latter series, conductivity decreased when the Al/Y substitution was increased, despite the number of charge carriers (Na+) increasing. Conductivity was 0.8 mS cm−1 when x = 0, and 0.08 mS cm−1 when x = 0.3. This demonstrates that the transport properties in NASICON materials are not only affected by the steric interactions (i.e. reff, mean width of conduction path) but also the electrostatic interactions (charge, bond strength, atomic orbitals) of the substituting cations.