Reaction energetics and crystal structure of Li4BN3H10 from first principles

Abstract

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.