  Reversible reaction involving Li amide (LiNH$_{2}$) and Li imide (Li$_{2}$NH)
is a potential mechanism for hydrogen storage. Recent synchrotron x-ray
diffraction experiments [W. I. David et al., J. Am. Chem. Soc. 129, 1594
(2007)] suggest that the transformation between LiNH$_{2}$ and Li$_{2}$NH is a
bulk reaction that occurs through non-stoichiometric processes and involves the
migration of Li$^{+}$ and H$^{+}$ ions. In order to understand the atomistic
mechanisms behind these processes, we carry out comprehensive first-principles
studies of native point defects and defect complexes in the two compounds. We
find that both LiNH$_{2}$ and Li$_{2}$NH are prone to Frenkel disorder on the
Li sublattice. Lithium interstitials and vacancies have low formation energies
and are highly mobile, and therefore play an important role in mass transport
and ionic conduction. Hydrogen interstitials and vacancies, on the other hand,
are responsible for forming and breaking N$-$H bonds, which is essential in the
Li amide/imide reaction. Based on the structure, energetics, and migration of
hydrogen-, lithium-, and nitrogen-related defects, we propose that LiNH$_{2}$
decomposes into Li$_{2}$NH and NH$_{3}$ according to two competing mechanisms
with different activation energies: one mechanism involves the formation of
native defects in the interior of the material, the other at the surface. As a
result, the prevailing mechanism and hence the effective activation energy for
decomposition depend on the surface-to-volume ratio or the specific surface
area, which changes with particle size during ball milling. These mechanisms
also provide an explanation for the dehydrogenation of LiNH$_{2}$+LiH mixtures.
