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We present a systematic first-principles investigation of the high-pressure structural stability of Li 2 BeH 4 . Our total-energy calculations show that at ambient pressure, the structure of ␣-Li 2 BeH 4 observed in experiments is more stable than the other proposed structures in this work and the structural transformation from ␣ to ␤ ͑Cs 2 MgH 4 type; Pnma͒ occurs at 18.1 GPa, together with a volume reduction of 4.7%. A detailed study of their electronic structures under ambient pressure up to 30.0 GPa reveals that this behavior is closely related to the variation in the Be-H covalent bonding in the BeH 4 anionic subunits of Li 2 BeH 4 . Based on a colligated analysis of the covalent bond number per unit area ͑N a ͒ and the scaled bond overlap population ͑BOP s ͒, ␤-NaAlH 4 and ␤-Mg͑AlH 4 ͒ 2 are expected to be the most promising candidates for hydrogen storage among the other investigated materials. However, the improvement of hydrogen absorption and/or desorption for Li 2 BeH 4 is less significant.

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By use of the linear-combination-of-atomic-orbital ͑LCAO͒ method for a cluster model, we studied the electronic structure of ␥-iron ⌺ 11 ͓11 0͔(113 ) grain boundary doping with N and Mn atoms. The effect of the segregation on the cohesion of the grain boundary is investigated based on the Rice-Wang thermodynamic model. It is found that N could not only largely enhance the cohesion of the grain boundary but also eliminate the detrimental effect of Mn. The cosegregation effect of Mn and N on the cohesion of the grain boundary depends on where they segregate. Nitrogen could be reliably used in alloyed steels as an efficient strengthening element.

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