  Using density functional theory we examine the crystal structure and the
finite-temperature thermodynamics of formation and dehydrogenation for the new
quaternary hydride Li4BN3H10. Two recent studies based on X-ray and neutron
diffraction have reported three bcc crystal structures for this phase. While
these structures possess identical space groups and similar lattice constants,
internal coordinate differences result in bond length discrepancies as large as
0.2 A. Geometry optimization calculations on the experimental structures reveal
that the apparent discrepancies are an artifact of X-ray interactions with
strong bond polarization; the relaxed structures are essentially identical.
Regarding reaction energetics, the present calculations predict that the
formation reaction 3LiNH2 + LiBH4 -> Li4BN3H10 is exothermic with enthalpy
Delta H(T=300K) = -11.8 kJ/(mol f.u.), consistent with reports of spontaneous
Li4BN3H10 formation in the literature. Calorimetry experiments have been
reported for the dehydrogenation reaction, but have proven difficult to
interpret. To help clarify the thermodynamics we evaluate the free energies of
seventeen candidate dehydrogenation pathways over the temperature range T =
0-1000 K. At temperatures where H2 release has been experimentally observed (T
\~ 520-630 K), the favored dehydrogenation reaction is Li4BN3H10 -> Li3BN2 +
LiNH2 + 4 H2, which is weakly endothermic [Delta H(T=550K) = 12.8 kJ/(mol H2)].
The small calculated Delta H is consistent with the unsuccessful attempts at
re-hydriding reported in the literature, and suggests that the moderately high
temperatures needed for H-desorption result from slow kinetics.
