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.

Automated summary

The research reveals that at high pressure, sodium, silicon, and hydrogen can form superionic compounds, with Na2SiH6 being dynamically stable at low pressure and becoming superionic at 5 GPa, re-entering solid/fluid states at about 25 GPa. This observation of H- transport opens up the possibility of new H- conductors and has implications for the formation of conducting layers in carbon exoplanets, potentially enhancing their habitability.