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

In search of a high-capacity hydrogen storage system, we have investigated the thermodynamic properties of borohydrides [M(BH4)n]. Using positive muon-spin rotation and relaxation (μ+SR), we have acquired experimental data for a wide range of different powder samples below ambient temperature. Zero-field μ+SR measurements indicate the formation of the H-μ+-H system in LiBH4, NaBH4, KBH4 and Ca(BH4)2, but not in Mg(BH4)2. It is also found that the amplitude of the HμH signal (AHμH) varies with the electronegativity (χP) of Mn+. This is because, when χP of Mn+ is small, [BH4]- should be more negative, resulting in an increase in the electron-density of H- ions. Therefore, AHμH increases with decreasing χP. Since the thermodynamic stability of M(BH4)n also depends on χP, AHμH is thought to be a microscopic indicator for the stability of borohydrides.