Crystal Structure of the Solid Electrolyte [(CH3)4N]2Ag13I15

The crystal structure of ${\mathrm{[(CH}}_3{)}_4{\mathrm{N]}}_2{\mathrm{Ag}}_{13}{I}_{15}$ has been determined from single crystal x‐ray diffraction counter data. The structure belongs to space group ${\text{R32‐D}}_3^7$ and contains one (three) [(CH₃)₄N]₂Ag₁₃I₁₅ in a rhombohedral (hexagonal) unit cell with dimensions $\text{a\hspace{0.17em}=\hspace{0.17em}11.52\hspace{0.17em}±\hspace{0.17em}0.02 Å}$ , $\text{α\hspace{0.17em}=\hspace{0.17em}67.35\hspace{0.17em}±\hspace{0.17em}0.15°}$ $\text{(a\hspace{0.17em}=\hspace{0.17em}12.77\hspace{0.17em}±\hspace{0.17em}0.03, c\hspace{0.17em}=\hspace{0.17em}26.54\hspace{0.17em}±\hspace{0.17em}0.05 Å)}$ . All iodide ions are at the corners of face‐sharing tetrahedra. These tetrahedra are arranged in two kinds of channels, one of which ends at the crystal surfaces; the other is a 10‐tetrahedra channel beginning at one tetramethylammonium ion and ending at another. The former are probably the more important for the diffusion of the Ag⁺ ions through the crystal. The Ag⁺ ions may diffuse from one channel to another via interconnecting face‐sharing tetrahedra. One out of eight crystallographically nonequivalent tetrahedra is not in either kind of channel, but also links channels. There are 123 tetrahedra and 39 Ag⁺ ions in each triply primitive hexagonal cell. The distribution of Ag⁺ ions over the tetrahedral sites is significantly nonrandom. The lower symmetry of [(CH₃)₄N]₂Ag₁₃I₁₅, the larger volume required for the [(CH₃)₄N]⁺ ions, and the smaller ratio of iodide tetrahedra to Ag⁺ ions are probably among the reasons that the average ionic conductivity of [(CH₃)₄N]₂Ag₁₃I₁₅ is lower than that of RbaG₄I₅.