Hydrogenation Reaction Pathways in the Systems Li3N-H2, Li3N-Mg-H2, and Li3N-MgH2-H2 by in Situ X-ray Diffraction, in Situ Neutron Diffraction, and in Situ Thermal Analysis

The phase diagram Li-Mg-N-H offers ample opportunities for potential hydrogen storage systems. Three systems based on lithium nitride, Li3N, were investigated by time-resolved in situ methods (thermal analysis, Xray and neutron diffraction) at temperatures up to 703 K and hydrogen gas pressures up to 9.4 MPa. Pure lithium nitride reacts in a one-step reaction to lithium amide according to Li3N + 2H(2) -> LiNH2 + 2LiH at 1.0 MPa hydrogen pressure. Equimolar mixtures of lithium nitride with magnesium hydride, both at 1.5 and 9.4 MPa hydrogen gas pressure, react in the same way up to 543 K, i.e., magnesium hydride does not participate in the reaction. At higher temperatures, lithium magnesium nitride is formed according to the endothermic reaction LiNH2 + MgH2 -> LiMgN + 2H(2) at moderate (<= 1.5 MPa) and via the exothermic reaction Li3N + MgH2 -> LiMgN + 2LiH at higher hydrogen gas pressures (5.0 MPa). Mixed imide Li2Mg(NH)(2) is formed when an excess of Li3N is used in the reaction. The hydrogenation of mixtures of lithium nitride with magnesium starts with the formation Li2NH and Li4NH, followed by the mixed imide Li2Mg(NH)(2) at higher temperatures and finally the formation of Mg3N2 and LiH. Deuterides react accordingly.