Effect of the lattice volume on the Al3+ ion conduction in NASICON type solid electrolyte

New trivalent Al3+ ion conductors with NASICON type structure, [Al0.2(Zr1–xTix)0.8]20/19Nb(PO4)3 and (Al1–xLax)4/19(1–x)Zr(16–19x)/19(1–x)Nb(PO4)3, were developed and their ion conducting properties were investigated. The lattice size of the NASICON type (AlxZr1–x)4/(4-x)Nb(PO4)3 series was successfully controlled by intentionally doping Ti4+ ion or La3+ ion in the crystal lattice, and the most suitable lattice size for the Al3+ ion conduction was identified from the relationship between the Al3+ ion conductivity and the lattice volume. The highest ion conductivity of 6.1 × 10− 4 S·cm− 1 at 600 °C was obtained for [Al0.2(Zr0.8Ti0.2)0.8]20/19Nb(PO4)3 among the NASICON type phosphate series prepared. Since the Al3+ ion conduction was dependent upon the crystal lattice size, it was demonstrated that the crystal lattice volume can be one of the key factors to optimize the ion conduction in the NASICON type structure.