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K. Yvon

Hydrogen in novel solid-state metal hydrides

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⁺²₂ · [NiH₄]^4– · 3H^–). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce⁴⁺ → Ce³⁺ valence change in CeMn_1.8Al_0.2H_4.4 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.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 218, 02/2003
Pages: 108

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