Density functional study ofLi4NHandLi1.5NH1.5as intermediary compounds during hydrogenation ofLi3N

Abstract

Recent experimental data suggest the formation of two new compounds, namely, ${\text{Li}}_{4}\text{NH}$ and ${\text{Li}}_{1.5}{\text{NH}}_{1.5}$, during the hydrogenation process of ${\text{Li}}_{3}\text{N}$. The formation of these compounds could modify the hydrogen absorption and desorption characteristics of ${\text{Li}}_{3}\text{N}$. We present here the results of our density functional theory calculations concerning their formation. We find that the direct hydrogenation reaction of ${\text{Li}}_{3}\text{N}$ to ${\text{Li}}_{2}\text{NH}$ is predominantly favored but the formation of ${\text{Li}}_{4}\text{NH}$ is possible through the direct formation involving ${\text{Li}}_{3}\text{N}$ and LiH with an enthalpy of reaction much less negative than for the direct formation of ${\text{Li}}_{2}\text{NH}$. The formation of this compound through the release of ammonia is not possible. This compound readily reacts with ${\text{H}}_{2}$ exothermically with an enthalpy of reaction less negative than for the direct process. We also find that the formation of the intermediate phase ${\text{Li}}_{2\ensuremath{-}x}{\text{NH}}_{1+x}$ for $x=0.5$ between imide $(x=0)$ and amide $(x=1)$ is possible. ${\text{Li}}_{1.5}{\text{NH}}_{1.5}$ is found to form in a cubic Li-vacant-type compound. After full relaxations of several structural models, the ${\text{Li}}_{1.5}{\text{NH}}_{1.5}$ compound presents a coexistence of ordered ${[\text{NH}]}^{2\ensuremath{-}}$ and ${[{\text{NH}}_{2}]}^{\ensuremath{-}}$ anions. These results are discussed in terms of an analysis of the electronic structures of these compounds.