Study of the Crystal Structure and Phase Transition of Li 2 NH System
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1098-HH03-06
Study of the Crystal Structure and Phase Transition of Li2NH System Jinbo Yang1, J. Lamsal2, Q Cai3, W B Yelon1, and W J James1 1 Materials Research Center, Missouri University of S&T, Rolla, MO, 65409 2 Physics Department, University of Missouri-Columbia, Columbia, MO, 65211 3 Physics Department, Universiy of Missouri-Columbia, Columbia, MO, 65211 ABSTRACT 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 αLi2NH 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 β- and γ-Li2NH phases. α-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. INTRODUCTION In order to achieve the targets for mobile hydrogen fuel cell applications, it is important to develop hydrogen carrying systems with a high weight percentage and a high capacity of hydrogen that can be readily released and recharged. Recently, Li-N-H has attracted much attention as a hydrogen storage material.[1] Lithium nitride Li3N absorbs hydrogen in a two step reaction to form LiNH2 + LiH, with a theoretical hydrogen capacity of 10.4 wt %. However, its reversible hydrogen capacity is only about 5.5% due to the fact that only the second step, Li2NH + LiH+ H2 = LiNH2 + 2LiH, is reversible under practical conditions. It is found that LiNH2 plays a critical role in the hydrogen cycling. LiNH2/Li3N mixtures exhibit a high reversible hydrogen capacity of 10%.[2] Recently compounds formed from stoichiometic mixtures of LiNH2 and LiBH4 in ratios of 1:1, 2:1 and 3:1 have been shown to form compounds that contain large quantities of hydrogen as high as 11.9 wt% in the case of Li4BN3H10, and therefore may be useful. [2-4]. The performance of Li2NH as a hydrogen storage material is strongly dependent on the preparative methods. It was reported that Li2NH prepared via reaction between Li3N and Li2NH shows much higher reversible hydrogen storage capacity with fast kinetics and excellent stability as compared to those prepared via the conventional LiNH2 decomposition method. However, the mechanism of the hydrogenation/dehydrogenation in the Li-N-H system is still not clear, in large part owing to the lack of accurate crystal structures of LiNH2 and Li2NH. The crystal structure of Li2NH has been reported as an anti-fluorite type structure (space group Fm 3 m ) consisting of Li+ an
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