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

The crystal structure of [(CH3)4N]2Ag13I15 has been determined from single crystal x-ray diffraction counter data. The structure belongs to space group R32-D37 and contains one (three) [(CH3)4N]2Ag13I15 in a rhombohedral (hexagonal) unit cell with dimensions a = 11.52 ± 0.02 Å, α = 67.35 ± 0.15° (a = 12.77 ± 0.03, c = 26.54 ± 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.