Na⁺ ion conductivity and crystallographic cell characterization in the Hf-nasicon system Na_1+xHf₂Si_xP_3-xO₁₂

We have measured the effect of varying the mobile ion concentration on the sodium ion conductivity in the Hf-Nasicon system, Na_1+xHf₂Si_xP_3-xO₁₂, for 1.4 ≤ x ≤ 2.8. The conductivity is greatest for Na_3.2Hf₂Si_2.2 P_0.8O₁₂: σ_25°C = 2.3 × 10^-3 (ω cm)^-1, and σ_250°C = 1.7 × 10^-1 (ω cm)^-1. These values are approximately 50% greater and worse, respectively, than the values reported for the best Zr-Nasicon. We have characterized the variation of lattice parameters with composition and found the behavior to be similar to that of Zr-Nasicon. A small distortion from rhombohedral to monoclinic symmetry occurs for compositions 1.8 ≤ x ≤ 2.2.