Quasielastic neutron scattering probing H− dynamics in the H− conductors LaH3−2xOx

Abstract

Using an incoherent quasielastic neutron scattering (QENS) technique, we investigate ${\mathrm{H}}^{\ensuremath{-}}$ dynamics in a series of oxyhydrides $\mathrm{La}{\mathrm{H}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$ that exhibit characteristic high ${\mathrm{H}}^{\ensuremath{-}}$ conductivity. In the end member $\mathrm{La}{\mathrm{H}}_{3}$ ($x$ = 0), two kinds of ${\mathrm{H}}^{\ensuremath{-}}$ dynamics are identified: the jump diffusion and the localized motion. The jump length in the jump diffusion mode increases with increasing $T$. The localized motion is identified as a jump between the two inequivalent sites. These dynamics are corroborated by our molecular dynamical simulations. Our QENS data suggest that similar ${\mathrm{H}}^{\ensuremath{-}}$ dynamics occurs also in oxyhydrides $\mathrm{La}{\mathrm{H}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$ ($x\ensuremath{\ne}$ 0), whose ${\mathrm{H}}^{\ensuremath{-}}$ concentration dependence is consistent with the previous measurement of ionic conductivity. We also discuss the possibility that $\mathrm{La}{\mathrm{H}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$ is an example of ${\mathrm{H}}^{\ensuremath{-}}$ ion conductors governed by the concerted migration mechanism. The identified ${\mathrm{H}}^{\ensuremath{-}}$ dynamics is key to understanding the anomalous hydrogen concentration dependence of the diffusion coefficient in lanthanum hydrides, which has been a longstanding mystery in this compound.