By Kari Williamson

The structures are said to contain a previously-unexplored combination of rare-earth and d-transition metals ideally suited to the compact storage of hydrogen.

Hydrogen in its natural gaseous state takes up too much space to store and transport efficiently, but using metal hydrides, metallic compounds that incorporate hydrogen atoms as a storage medium for hydrogen could solve this problem.

The metal hydrides bind to hydrogen to produce a solid one thousand times or more smaller than the original hydrogen gas. The hydrogen can then later be released from the solid by heating it to a given temperature.

The heterometallic hydride clusters synthesised by the RIKEN researchers use rare-earth and d-transition metals as building blocks and exploit the advantages of both.

Rare earth metal hydrides remove one major obstacle by enabling analysis using X-ray diffraction, a technique which is infeasible for most other metal hydrides – offering unique insights into underlying reaction processes involved, the researchers say.

Rare earth metal hydrides on their own, however, do not undergo reversible hydrogen addition and release, the cornerstone of hydrogen storage. This becomes possible through the addition of a d-transition metal, in this case tungsten (W) or molybdenum (Mo).

The current research is said to be the first to explore complexes with multiple rare earth atoms of the form Ln4MHn and with well-defined structures (Ln = a rare-earth metal such as yttrium, M = a d-transition metal, either tungsten or molybdenum, and H = hydrogen).