4. Polyanionic hydrides

Metal hydrides are intensively investigated in the quest for useful hydrogen storage materials. The general goal of this research line is to systematically study polyanionic hydrides which can be obtained from the hydrogenation of Zintl phases. Zintl phases consist of an electropositive metal (e.g. alkaline earth metals) and an electronegative metal or semimetal (the E component, e.g. Al , Ga , Ge, Si, P, As). These phases are distinguished by the presence of a polyanionic framework formed by the E metals, which is counterbalanced by the oxidised electropositive metal. In polyanionic hydrides H is incorporated in the polyanion of Zintl phases and acts as a terminating ligand which creates novel arrangements containing both E-E and E-H bonds. This research line was triggered by the report of the synthesis of the hydride SrAl 2H 2 by Gingl et al in 2000. The structure of this remarkable compound features polyanionic two-dimensional nets [Al 2H 2] 2- [ F. Gingl, T. Vogt, E. Akiba, J. Alloys and Compd., 306 (2000), 127 ].

We are specifically interested to explore which elements E or combinations of E elements form polymeric anions with hydrogen and how hydride formation is influenced by the choice of the countercation. For example, in SrAlSiH hydrogen is exclusively attached to Al and part of a layered polyanion [AlHSi] 2-. The material represents the first narrow-gap semiconductor hydride and combines the high thermal stability of saline hydrides with the air and moisture stability of interstitial transition metal hydrides.

Fig. 5: (a) Crystal structure of SrAlSiH. (b) Optical absorption spectrum from 0.4 to 1.35 eV for SrAlSiH and SrAlSiD measured at room temperature. (c) Electronic density of states (DOS) for SrAl 2H 2 and SrAlSiH. (d) Band structure of SrAlSiH. The Si-p z orbital contribution is shown as fat-band representation.

T. Björling, D. Noréus, K. Jansson, M. Andersson, E. Leonova, M. Edén, U. Hålenius, U. Häussermann “SrAlSiH: A Polyanionic Semiconductor Hydride” Angew. Chem. Int. Ed., 44 (2005), 7269.