Investigation of the dynamics in KSiH3 and new bi-anionic K(SiH3)1-x(BH4)x complex hydrides by MAS-NMR spectroscopy

Abstract

Bi-anionic K(SiH3)1-x(BH4)x compounds were synthesized for the first time by ball-milling from mixtures of KBH4 and KSiH3 followed by annealing at 200°C under H2. A full solid-solution following the Vegard’s law was found in the binary KSiH3-KBH4 phase diagram with all compounds crystallizing in the cubic structure with Fm-3m space group. In-depth MAS-NMR spectroscopy characterizations were carried out confirming undoubtedly that the two types of anions coexist in the same phase. Interestingly, the progressive introduction of [BH4]- in the KSiH3 compound led to an evolution from the very fast orientationally diffusive rotation of [SiH3]- units to a more strongly restricted motion. Upon thermal treatment, the K(SiH3)1-x(BH4)x compounds desorbed only hydrogen atoms from the [SiH3]- anion around 180°C, which limits their potential use for hydrogen storage. Further works on the combination of the [SiH3]- anion with other hydrogen-containing polyanions could allow the design of new complex hydrides for hydrogen storage.