Substitution of 5% of Zr by Ca in LiZr2(PO4)3 transforms the structure to that of rhombohedral NASICON to give a room-temperature bulk Li-ion conductivity σ Li ≈1.2×10−4 Scm−1, which is comparable to that of Li1.3Ti1.7Al0.3(PO4)3 now being used as a solid Li-ion separator in test cells of novel Li-ion batteries.