Pressure-Induced Superionicity of H^- in Hypervalent Sodium Silicon Hydrides

Superionic states simultaneously exhibit properties of a fluid and a solid. Proton (H⁺) superionicity in ice, H₃O, He-H₂O, and He-NH₃ compounds is well-studied. However, hydride (H^-) superionicity in H-rich compounds is rare, being associated with instability and strongly reducing conditions. Silicon, sodium, and hydrogen are abundant elements in many astrophysical bodies. Here, we use first-principles calculations to show that, at high pressure, Na, Si, and H can form several hypervalent compounds. A previously unreported superionic state of Na₂SiH₆ results from unconstrained H^- in the hypervalent [SiH₆]^2- unit. Na₂SiH₆ is dynamically stable at low pressure (3 GPa), becoming superionic at 5 GPa, and re-entering solid/fluid states at about 25 GPa. Our observation of H^- transport opens up a new field of H^- conductors. It also has implications for the formation of conducting layers at depth in exotic carbon exoplanets, potentially enhancing the habitability of such planets.