High resolution transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction studies of nanocrystalline manganese borohydride (Mn(BH4)2) after mechano-chemical synthesis and thermal dehydrogenation

In order to synthesize manganese borohydride, Mn(BH4)2, mechano-chemical activation synthesis (MCAS) of the (2LiBH4 +MnCl2) powder mixture was carried out by ball milling in a magneto ball mill. Both X-ray diffraction and TEM selected area electron diffraction patterns (SAEDPs) clearly confirm the presence of the Mn(BH4)2 and LiCl phases in the synthesized nanocomposite. No other phases were detected. Bright field high-resolution TEM imaging of the synthesized composite powder particles reveals the presence of nanograins consistent with LiCl and Mn(BH4)2 within the powder particles. Their respective grain sizes, estimated as the equivalent circle diameters (ECD) from the high-resolution TEM micrographs with the corrected sample standard deviations, are within the range of 14.1±3.7nm and 10.0±2.9nm for LiCl and Mn(BH4)2, respectively. The XRD patterns of the thermally dehydrogenated (Mn(BH4)2 +2LiCl) nanocomposite do not exhibit any Bragg diffraction peaks belonging to either crystalline Mn or B. In contrast, the SAED patterns and EDS elemental maps provide strong evidence that both Mn and B exist in the dehydrogenated powder as crystalline phases α-Mn and β-B, respectively. The results show that the lack of XRD Bragg diffraction peaks is insufficient evidence that the Mn and B elemental products of Mn(BH4)2 thermolysis can be classified as being amorphous.