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Analysis of Dihydrogen Bonding in Ammonium Borohydride

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  • Stanislav Filippov, Linköping University, Stockholm University
  • ,
  • Jakob B. Grinderslev
  • Mikael S. Andersson, Chalmers University of Technology
  • ,
  • Jeff Armstrong, Rutherford Appleton Laboratory
  • ,
  • Maths Karlsson, Chalmers University of Technology
  • ,
  • Torben R. Jensen
  • Johan Klarbring, Linköping University
  • ,
  • Sergei I. Simak, Linköping University
  • ,
  • Ulrich Häussermann, Stockholm University

The structural and vibrational properties of ammonium borohydride, NH4BH4, have been examined by first-principles density functional theory (DFT) calculations and inelastic neutron scattering (INS). The H disordered crystal structure of NH4BH4 is composed of the tetrahedral complex ions NH4 + and BH4 -, which are arranged as in the fcc NaCl structure and linked by intermolecular dihydrogen bonding. Upon cooling, the INS spectra revealed a structural transition between 45 and 40 K. The reversible transition occurs upon heating between 46 and 49 K. In the low-Temperature form reorientational dynamics are frozen. The libration modes for BH4 - and NH4 + are near 300 and 200 cm-1, respectively. Upon entering the fcc high-Temperature form, NH4 + ions attain fast reorientational dynamics, as indicated in the disappearance of the NH4 + libration band, whereas BH4 - ions become significantly mobile only at temperatures above 100 K. The vibrational behavior of BH4 - ions in NH4BH4 compares well to the heavier alkali metal borohydrides, NaBH4-CsBH4. DFT calculations revealed a nondirectional nature of the dihydrogen bonding in NH4BH4, with only weak tendency for long-range order. Different rotational configurations of complex ions appear quasi-degenerate, which is reminiscent of glasses.

TidsskriftJournal of Physical Chemistry C
Sider (fra-til)28631-28639
Antal sider9
StatusUdgivet - nov. 2019

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