Effect of Homovalent Framework Cation Substitutions on the Sodium Ion Conductivity in Na ₃ Zr ₂ Si ₂ PO ₁₂

The reported activation volume for sodium ion transport in the fast‐ion conductor Na 3 Zr 2 Si 2 PO 12 is surprisingly large. Therefore, attempts were made to alter the size of the diffusion channels through partial substitution of As, Ge, Ti, Th, and Hffor the framework cations to decrease geometric structural hinderance of the diffusion process. Sodium ion conductivity is improved at temperatures below 200°C far the Hf substitutions. The existence and electrical properties of the apparently isostructural Na 3 Hf 2 Si 2 PO 12 , which has a room‐temperature conductivity of 1.0Ω10 −3 Ω cm −1 , are reported.