posted on 2021-07-05, 04:43authored byJakob
B. Grinderslev, Mikael S. Andersson, Benjamin A. Trump, Wei Zhou, Terrence J. Udovic, Maths Karlsson, Torben R. Jensen
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.