The origins of ultra-low velocity zones (ULVZs) detected at Earth’s core-mantle boundary, potentially linked to large low shear velocity provinces (LLSVPs), have been a subject of ongoing debate. Recent experiments have demonstrated the formation of iron hydride through a water-iron reaction under lowermost-mantle conditions; however, the stability of this compound has been inadequately constrained. By integrating first-principles molecular dynamic simulations with a machine learning approach, we have determined the stability and elastic properties of iron hydride under core-mantle boundary conditions.

Our results reveal that iron hydride is a stable superionic phase, in which hydrogen diffuses akin to a liquid while iron vibrates at lattice sites. Significantly, this phase exhibits markedly slower velocities and a higher density compared to the ambient mantle under lowermost-mantle conditions. The accumulation of iron hydride, through either water-iron reaction or the solidification of core material, provides a plausible explanation for seismic observations of ULVZs, particularly those associated with subduction. This work underscores the substantial role of water in generating seismic heterogeneities at the core-mantle boundary.