Synthesis, Characterization, and Polymorphism of [H₃O][NbF₆]: A Polar and Possibly Ferroelectric Oxonium Salt

Abstract [H 3 O][NbF 6 ] was obtained from the controlled hydrolysis of NbF 5 in anhydrous liquid HF. It adopts a polar, orthorhombic crystal structure with space group Iba 2 (no. 45, oI 88) at room temperature. A first‐order phase transition at 137 K leads to a cubic non‐centrosymmetric polymorph in space group I 2 1 3 (no. 199, cI 88). This low‐temperature modification results from a distinct rotation of the [H 3 O] + cations canceling their polar orientation in the room temperature phase. Quantum‐chemical calculations estimate a rotational barrier between 5.8 to 6.4 kJ/mol. At a temperature of 363 K, the compound adopts a centrosymmetric, cubic crystal structure in space group Pm m (no. 221, cP 11) that shows rotational disorder of cations and anions. The transition from the polar phase at room temperature to the centrosymmetric phase at high temperature not only reveals the plastic nature of the high‐temperature structure but also hints at potential ferroelectric properties, underscoring the multifaceted behavior of [H 3 O][NbF 6 ] across different temperature regimes.