Dehydriding process and hydrogen-deuterium exchange of LiBH₄-Mg₂FeD₆ composites

The dehydriding process and hydrogen-deuterium exchange (H-D exchange) of xLiBH₄ + (1 - x)Mg₂FeD₆ (x = 0.25, 0.75) composites has been studied in detail. For the composition with x = 0.25, only one overlapping mass peak of all hydrogen and deuterium related species was observed in mass spectrometry. This implied the simultaneous dehydriding of LiBH₄ and Mg₂FeD₆, despite an almost 190 °C difference in the dehydriding temperatures of the respective discrete complex hydrides. In situ infrared spectroscopy measurements indicated that H-D exchange between [BH₄]^- and [FeD₆]^4- had occurred during ball-milling and was promoted upon heating. The extent of H-D exchange was estimated from the areas of the relevant mass signals: immediately prior to the dehydriding, more than two H atoms in [BH₄]^- was replaced by D atoms. For x = 0.75, H-D exchange also occurred and about one to two H atoms in [BH₄]^- was replaced by D atoms immediately before the dehydriding. In contrast to the situation for x = 0.25, firstly LiBH₄ and Mg₂FeD₆ dehydrided simultaneously with a special molar ratio = 1:1 at x = 0.75, and then the remaining LiBH4 reacted with the Mg and Fe derived from the dehydriding of Mg₂FeD₆.