Geometric Frustration and Concerted Migration in the Superionic Conductor Barium Hydride

Ionic conductivity is a phenomenon of great interest,
not least
because of its application in advanced electrochemical devices such
as batteries and fuel cells. While lithium, sodium, and oxide fast
ion conductors have been the subjects of much study, the advent of
hydride (H–) ion fast conductors opens up new windows
in the understanding of fast ion conduction due to the fundamental
simplicity of the H– ion consisting of just two
electrons and one proton. Here we probe the nature of fast ion conduction
in the hydride ion conductor, barium hydride (BaH2). Unusually
for a fast ion conductor, this material has a structure based upon
a close-packed hexagonal lattice, with important analogues such as
BaF2 and Li2S. We elucidate how the structure
of the high temperature phase of BaH2 results in a disordered
hydride sublattice. Furthermore, using novel combined quasi-elastic
neutron scattering (QENS) and electrochemical impedance spectroscopy
(EIS) we show how the high energy ions interact to create a concerted
migration that results in macroscopic superionic conductivity via
an interstitialcy mechanism.