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(BD)·3ND 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 -Y(BD)·3ND was discovered. Furthermore, the series of Y(BH)·NH ( = 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 BH and NH groups and unique insights into the hydrogen dynamics. Partial deuteration using ND in Y(BH)·ND ( = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH and BH in Y(BH)·NH, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(BH)·NH could roughly be divided into three regions: (i) below 55 meV, containing mainly BH librational motions, (ii) 55-130 meV, containing mainly NH librational motions, and (iii) above 130 meV, containing B-H and N-H bending and stretching motions.