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, HO, He-HO, and He-NH 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 NaSiH results from unconstrained H in the hypervalent [SiH] unit. NaSiH 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.