A stepwise phase transition in the formation of lithium amidoborane via the solid-state reaction of lithium hydride and ammonia borane has been identified and investigated Structural analyses reveal that a lithium amidoborane-ammonia borane complex (LiNH2BH3 center dot NH3BH3) and two allotropes of lithium amidoborane (denoted as alpha- and beta-LiNH2BH3, both of which adopt orthorhombic symmetry) were formed in the process of synthesis LiNH2BH3 center dot NH3BH3 is the intermediate of the synthesis and adopts a monoclinic structure that features layered LINH2BH3 and NH3BH3 molecules and contains both ionic and dihydrogen bonds Unlike alpha-LINH2BH3, the units of the beta phase have two distinct Li+ and [NH2BH3](-) environments. beta-LINH2BH3 can only be observed in energetic ball milling and transforms to alpha-LiNH2BH3 upon extended milling. Both allotropes of LiNH2BH3 exhibit similar thermal decomposition behavior, with 10.8 wt % H-2 released when heated to 180 degrees C; in contrast, LINH2BH3 center dot NH3BH3 releases approximately 14.3 wt % H-2 under the same conditions