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Reorientational Dynamics in Y(BH4)3·xNH3 (x = 0, 3, and 7): The Impact of NH3 on BH4- Dynamics

J B Grinderslev1, U Häussermann2, T R Jensen1

  • 1Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Aarhus DK-8000, Denmark.

The Journal of Physical Chemistry. C, Nanomaterials and Interfaces
|March 27, 2024
PubMed

Insights

Investigating Y(BH4)3·xNH3 with neutrons reveals how NH3 ligands control BH4- anion mobility. This impacts ionic conductivity in related materials.

Area of Science:

  • Materials Science
  • Solid-state Chemistry
  • Neutron Scattering Spectroscopy

Background:

  • Understanding the dynamics of anions and ligands in metal borohydrides is crucial for developing advanced materials.
  • The influence of ammonia (NH3) ligands on the reorientational dynamics of the borohydride (BH4-) anion in Y(BH4)3·xNH3 systems is not fully understood.
  • Enhanced ionic conductivity in related materials like LiBH4·xNH3 suggests a link to molecular mobility.

Purpose of the Study:

  • To elucidate the reorientational dynamics of the BH4- anion and NH3 ligands in Y(BH4)3·xNH3 (x = 0, 3, 7).
  • To investigate the effect of varying NH3 ligand stoichiometry on the mobility of the BH4- anion.
  • To establish structure-dynamics-conductivity relationships in metal borohydride-ammonia systems.

Main Methods:

  • Quasielastic Neutron Scattering (QENS) was employed to study the reorientational motions.
  • Neutron Spin Echo (NSE) spectroscopy provided complementary data on dynamics over different timescales.
  • Analysis focused on determining the type of reorientation (e.g., 2-fold, 3-fold) and relaxation times.

Main Results:

  • The number of NH3 ligands significantly alters the reorientational mobility of the BH4- anion.
  • BH4- exhibits 2-fold reorientation in Y(BH4)3, 3-fold in Y(BH4)3·3NH3, and mixed 2-fold/3-fold in Y(BH4)3·7NH3.
  • Relaxation times of BH4- decrease dramatically with increasing NH3 content, indicating enhanced mobility.

Conclusions:

  • The addition of NH3 ligands enhances the reorientational dynamics of the BH4- anion in Y(BH4)3.
  • NH3 ligands themselves undergo 3-fold reorientation and quantum mechanical rotational tunneling.
  • These findings provide critical insights into the mechanism of enhanced ionic conductivity in related metal borohydride-ammonia systems.

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