Bi12Rh3Ag6I9: Mobile Silver Ions and Surface-Dominated Transport in a Layered Weak Topological Insulator

Bi12Rh3Ag6I9 extends the Bi14Rh3I9 family of layered bismuth-rich iodides in which intermetallic topological-insulator sheets alternate with salt-like spacer layers. High-temperature synthesis yields phase-pure powders and millimeter-sized platelets. Single-crystal X-ray diffraction reveals a rhombohedral structure composed of [Bi12Rh3I]2+ layers and [Ag6I8]2- spacers. In this spacer, Ag+ cations occupy 75% of the tetrahedral voids of a double iodide layer, giving rise to intrinsic two-dimensional cation disorder and mobility. Electrochemical impedance spectroscopy reveals a room-temperature ionic conductivity of about 10-6 S cm-1. Density-functional theory including spin-orbit coupling predicts band inversion and weak-topological-insulator invariants, while angle-resolved photoemission spectroscopy corroborates the calculated bulk electronic structure, yielding an experimental bulk band gap of 286 meV. Transport measurements show strongly anisotropic, initially n-type conduction. During the initial measurement cycle, applied bias reduces the electron doping through slow electrochemical deintercalation of silver. After conditioning, bulk conduction is strongly reduced; transport below about 130 K becomes nonactivated, the bias-induced drift ceases below about 50 K as Ag+ motion freezes, and a pronounced resistivity drop perpendicular to the layers below 3.2 K is suggestive of a boundary-sensitive low-dimensional transport regime. Bi12Rh3Ag6I9 demonstrates that spacer chemistry provides a chemical lever for carrier-density control in layered topological insulators.