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

J B Grinderslev; U Häussermann; T R Jensen; A Faraone; M Nagao; M Karlsson; T J Udovic; M S Andersson

doi:10.1021/acs.jpcc.4c00265

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

J Phys Chem C Nanomater Interfaces. 2024 Mar 9;128(11):4431-4439. doi: 10.1021/acs.jpcc.4c00265. eCollection 2024 Mar 21.

Authors

J B Grinderslev¹, U Häussermann², T R Jensen¹, A Faraone³, M Nagao^{3

4

5}, M Karlsson⁶, T J Udovic^{3

4}, M S Andersson⁷

Affiliations

¹ Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Aarhus DK-8000, Denmark.
² Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden.
³ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States.
⁴ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, United States.
⁵ Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States.
⁶ Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg SE-412 96, Sweden.
⁷ Ångström Laboratory, Department of Chemistry, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden.

Abstract

The reorientational dynamics of Y(BH₄)₃·xNH₃ (x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH₃ ligands drastically alters the reorientational mobility of the BH₄^- anion. From the QENS experiments, it was determined that the BH₄^- anion performs 2-fold reorientations around the C₂ axis in Y(BH₄)₃, 3-fold reorientations around the C₃ axis in Y(BH₄)₃·3NH₃, and either 2-fold reorientations around the C₂ axis or 3-fold reorientations around the C₃ axis in Y(BH₄)₃·7NH₃. The relaxation time of the BH₄^- anion at 300 K decreases from 2 × 10^-7 s for x = 0 to 1 × 10^-12 s for x = 3 and to 7 × 10^-13 s for x = 7. In addition to the reorientational dynamics of the BH₄^- anion, it was shown that the NH₃ ligands exhibit 3-fold reorientations around the C₃ axis in Y(BH₄)₃·3NH₃ and Y(BH₄)₃·7NH₃ as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH₄·xNH₃ and Mg(BH₄)₂·xCH₃NH₂.