Halogenated Sodium-<i>closo</i>-Dodecaboranes as Solid-State Ion Conductors

Sodium compounds containing large weakly coordinating anions are explored as ion conductors. The halogenated sodium-closo-dodecaboranes (Na2B12Cl12, Na2B12Br12, and Na2B12I12) are all isostructural (Pa3̅) at room temperature. These compounds undergo an order–disorder polymorphic transition to Fm3̅m where Na+ partially occupies two crystallographic sites and [B12X12]2– anions undergo reorientational motion. These dynamic structural properties promote extreme Na+ ion conductivity up to 0.162 S/cm at elevated temperature (500 °C). The polymorphic transition temperatures increase down the halogen group (Cl < Br < I). These temperatures are much higher (475, 525, and 570 °C) than for Na2B12H12 (266 °C), which could be related to increasing anion size, mass, and the anisotropic electron density in the covalently bound halogens (B–X). The halogens may form Na–X interactions with increasing strength and directionality, which restrict dynamic motion until high temperature. The halogenated sodium-closo-dodecaboranes demonstrate excellent thermal stabilities (up to 500 °C) and may facilitate the development of new high temperature ion conductors.