Magnesium based ternary metal hydrides containing alkali and alkaline-earth elements

A review of structurally characterised ternary metal hydrides containing magnesium and alkali (A) or alkaline-earth (Ae) elements (including divalent Eu and Yb) is given. There exist 28 such compounds that cover 18 different crystal structures. Most were obtained by solid-state reactions under moderate hydrogen gas pressure while some required hydrostatic pressure in the kbar range. The compounds are saline and have ordered crystal structures. Their hydrogen contents can be rationalized in terms of mono and divalent metal cations, and hydrogen anions. Many have fluorine analogues, and those containing calcium and strontium tend to have ytterbium and europium analogues, respectively. Magnesium usually adopts octahedral and rarely tetrahedral or pentagonal bipyramidal hydrogen configurations. The metal configurations around hydrogen are mostly octahedral and tetrahedral, but square pyramidal, trigonal bipyramidal, trigonal and linear configurations are also found. Compared to binary hydrides, the metal–hydrogen bonds are generally shortened which is reflected by a volume contraction. The hydrogen storage efficiencies for A metal compounds reach 6.0 wt.% and 88 g l −1 (NaMgH 3) and for Ae metal compounds 5.7 wt.% and 99 g l −1 (Ca 4Mg 3H 14). The compounds generally desorb hydrogen above 670 K at 1 bar. The europium containing hydrides order magnetically at temperatures below 19–32 K.