Synthesis and decomposition of perovskite-type hydrides, MMgH3 (MÂ =Â Na, K, Rb)

The perovskite-type hydrides, MMgH3 (M = Na, K, Rb) were synthesized by ball-milling. They decomposed at the temperatures of 673-723 K in several ways depending on M. Namely, NaMgH3 decomposed in the two-step reactions, NaMgH3 → NaH + Mg + H2 and NaH → Na + (1/2)H2, whereas KMgH3 decomposed in the single-step reaction, KMgH3 → K + Mg + (3/2)H2. However, RbMgH3 decomposed in the two-step reactions, but each reaction still remained uncertain. Judging from the experimental value of ΔH for MMgH3 (M = Na, K, Rb), the phase stability was found to change in the order, RbMgH3 > KMgH3 > NaMgH3, so that NaMgH3 is the most unstable among them.