Microscopic indicator for thermodynamic stability of hydrogen storage materials provided by positive muon-spin rotation

Abstract

In search of a high-capacity hydrogen storage system, we have investigated the thermodynamic properties of borohydrides $[M{({\text{BH}}_{4})}_{2}]$. Using positive muon-spin rotation and relaxation $({\ensuremath{\mu}}^{+}\text{SR})$, we have acquired data for five different powder samples below ambient temperature. Zero-field ${\ensuremath{\mu}}^{+}\text{SR}$ measurements indicate the formation of the $\text{H-}{\ensuremath{\mu}}^{+}\text{-H}$ system in ${\text{LiBH}}_{4}$, ${\text{NaBH}}_{4}$, ${\text{KBH}}_{4}$, and $\text{Ca}{({\text{BH}}_{4})}_{2}$ but not in $\text{Mg}{({\text{BH}}_{4})}_{2}$. It is also found that the amplitude of the $\text{H}\ensuremath{\mu}\text{H}$ signal $({A}_{\text{H}\ensuremath{\mu}\text{H}})$ varies with the electronegativity $({\ensuremath{\chi}}_{\text{P}})$ of ${M}^{n+}$. Since the thermodynamic stability of $M{({\text{BH}}_{4})}_{n}$ also depends on ${\ensuremath{\chi}}_{\text{P}}$, ${A}_{\text{H}\ensuremath{\mu}\text{H}}$ is thought to be a microscopic indicator for the stability of $M{({\text{BH}}_{4})}_{n}$.