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 (LiNH(2)BH(3).NH(3)BH(3)) and two allotropes of lithium amidoborane (denoted as alpha- and beta-LiNH(2)BH(3), both of which adopt orthorhombic symmetry) were formed in the process of synthesis. LiNH(2)BH(3).NH(3)BH(3) is the intermediate of the synthesis and adopts a monoclinic structure that features layered LiNH(2)BH(3) and NH(3)BH(3) molecules and contains both ionic and dihydrogen bonds. Unlike alpha-LiNH(2)BH(3), the units of the beta phase have two distinct Li(+) and [NH(2)BH(3)](-) environments. beta-LiNH(2)BH(3) can only be observed in energetic ball milling and transforms to alpha-LiNH(2)BH(3) upon extended milling. Both allotropes of LiNH(2)BH(3) exhibit similar thermal decomposition behavior, with 10.8 wt % H(2) released when heated to 180 degrees C; in contrast, LiNH(2)BH(3).NH(3)BH(3) releases approximately 14.3 wt % H(2) under the same conditions.