First-principles study of cation and hydrogen arrangements in the Li-Mg-N-H hydrogen storage system

Citation:
Wang, Y, Chou MY.  2007.  First-principles study of cation and hydrogen arrangements in the Li-Mg-N-H hydrogen storage system, Jul. Physical Review B. 76:6., Number 1

Abstract:

Recently it was discovered that a total of 5.6 wt. % H-2 could be released from the 1:2 mixture of lithium amide and magnesium hydride at temperatures as low as 150 degrees C. With a reaction enthalpy of 44 KJ/mol H-2, this system has high potential for on-board hydrogen storage applications. The fully desorbed product is believed to be a mixed lithium and magnesium imide Li2Mg(NH)(2). In this work, the crystal structure of this mixed imide is studied from total-energy density-functional calculations. Based on a recent experimentally established space group, possible ordered configurations are examined. Important local orderings are identified for the experimentally observed disordered phase at room temperature. These unique local arrangements are also connected with the observed structural transitions above room temperature. In addition, the local ordering in Mg(NH2)(2) is analyzed. The similarity and difference of local arrangements among hydrogen, cations, and vacancies are discussed for the three amide (imide) systems: LiNH2, Mg(NH2)(2), and Li2Mg(NH)(2). The identification of the cation and hydrogen local orderings are expected to facilitate the design of new mixed imides and amides as hydrogen storage materials with desired physical properties.

Notes:

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