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 Vegard's law was found in the binary KSiH₃–KBH₄ phase diagram with all compounds crystallizing in the cubic structure with the Fmm space group. In-depth MAS-NMR spectroscopy characterization studies 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 studies on the combination of the [SiH₃]⁻ anion with other hydrogen-containing polyanions could allow the design of new complex hydrides for hydrogen storage.