Hydrogen in novel solid-state metal hydrides

Abstract

Abstract Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca4Mg3H14, Ca19Mg8H54, Eu2MgD6, Eu6Mg7D26 and Eu2Mg3D10, it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH4]– and d-elements such as [IrH5]4– and [IrH4]5– in the complex hydrides LiBH4 and Mg6Ir2H11, respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi4H4 (= Nd3+Mg+2·[Ni4H4]5–) and LaMg2NiD7 (= La3+Mg+2 2·[NiH4]4–·3H–). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce4+ → Ce3+ valence change in CeMn1.8Al0.2H4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ~2.6 Å, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg3Ir to the red colored hydride Mg6Ir2H11. In this article recent work and some methodological aspects are highlighted.