Study of the Crystal Structure and Phase Transition of Li 2 NH System

Neutron diffraction at different temperatures has been used to study the crystal structure and possible phase transitions of Li2NH. It was found that the crystal structure and phase transition are related to the synthesis methods. A phase transition from the low temperature phase 16-350 K to the high temperature phase above 370 K has been confirmed for the ケ-Li2NH sample prepared by reacting Li3N with LiNH2. The Li2NH (β-Li2NH) prepared by decomposition of LiNH2 shows only the high temperature phase. The reaction of LiH+LiNH2 at 300°C for 12 h under vacuum produces some Li2NH (γ-Li2NH) with partially unreacted LiNH2 and LiH as impurities. There is no phase transition in the temperature range from 16 K to 400 K for the - and β-Li2NH phases.ケ-Li2NH exhibits a higher reversible hydrogen storage capacity and faster kinetics. The structural differences among the lithium imides may lead to different reaction mechanisms for hydrogen absorption/desorption in the Li-N-H system.