Lithium imide ͑Li 2 NH͒ has been considered as a promising medium for hydrogen storage with the following reaction: LiNH 2 + LiH↔ Li 2 NH+ H 2 . All possible phases involved in the reaction need to be fully characterized in order to understand the right pathway connecting the two end compounds LiNH 2 and Li 2 NH and to further improve its reaction condition to meet the requirements of practical applications. We study from first-principles calculations the possible intermediate compounds Li 2−x NH 1+x between Li 2 NH and LiNH 2 . Based on the energetics results, possible intermediate phases are identified for 0 Ͻ x Յ 1 / 4. On the other hand, the intermediate phases are not thermodynamically favorable for 3 / 4 Յ x Ͻ 1 with respect to phase separation into Li 2 NH and LiNH 2 . The NH and NH 2 anions coexist in the intermediate compounds, but the electronic states derived from these two units are well separated in energy. The band gap of the intermediate compounds is also smaller than that of both Li 2 NH and LiNH 2 . These signatures from the electronic structure will provide useful guidelines for experimental efforts to search for the intermediate phases.

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We report an experimental demonstration of electromagnetically induced transparency in the transient optical response in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells, and analyze the cascade-type three-level schemes with density matrix and Maxwell equations to then obtain the phase shift, group velocity, and group-velocity dispersion. The calculated group velocity is ∼6.87 × 10 4 m/s and the corresponding pulse delay is ∼7 ps. Finally, we provide a convenient basis for investigating many-body effects in semiconductors.