Is Earth's Inner Core in a Superionic State?

This article reviews recent progress in the high-pressure studies of iron hydride (FeHX). The superionic phase transition of FeHX has recently been predicted from ab initio calculations, and the transition may occur in the Earth's inner core. Approaches for its experimental verification are discussed based on existing studies of picosecond acoustics and electrical resistivity measurements of FeHX at high pressures generated using a diamond anvil cell. The development of solid-state ionics in high-pressure Earth and planetary sciences is anticipated.