Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(¹¹BD₄)₃·3ND₃ with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(¹¹BD₄)₃·3ND₃ was discovered. Furthermore, the series of Y(¹¹BH₄)₃·xNH₃ (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the ¹¹BH₄^- and NH₃ groups and unique insights into the hydrogen dynamics. Partial deuteration using ND₃ in Y(¹¹BH₄)₃·xND₃ (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH₃ and ¹¹BH₄^- in Y(¹¹BH₄)₃·xNH₃, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(¹¹BH₄)₃·xNH₃ could roughly be divided into three regions: (i) below 55 meV, containing mainly ¹¹BH₄^- librational motions, (ii) 55-130 meV, containing mainly NH₃ librational motions, and (iii) above 130 meV, containing ¹¹B-H and N-H bending and stretching motions.