Stabilization of ammonium borohydride in solid solutions of NHBH-MBH (M = K, Rb, Cs).

Ammonium borohydride, NHBH, has the highest gravimetric and volumetric hydrogen density among known inorganic compounds and a fascinating rock salt type crystal structure composed of H disordered tetrahedral complexes, NH and BH, which are interlinked by a dense network of dihydrogen bonds. Here we report the synthesis, structure and properties of solid solutions in the binary systems, NHBH-MBH (M = K, Rb, Cs), which are investigated by synchrotron radiation powder X-ray diffraction and thermal and photographic analysis. Full solubility and formation of (NH)MBH, is observed upon cryo-mechanochemical treatment. The solid solutions stabilize NHBH from ∼68 to ∼96 °C, alter the decomposition pathway and suppress the fierce decomposition of NHBH. However, for increased amounts of NHBH in the solid solutions, the decomposition gradually shows more resemblance to that of pristine ammonium borohydride, and the thermal stability of the solid solutions appears to decrease down the group of the alkali metal ions, decreasing from K, Rb and to Cs.