Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Structural,
vibrational, and dynamical properties of the mono-
and mixed-alkali silanides (MSiH3, where M = K, Rb, Cs,
K0.5Rb0.5, K0.5Cs0.5,
and Rb0.5Cs0.5) were investigated by various
neutron experiments, including neutron powder diffraction (NPD), neutron
vibrational spectroscopy (NVS), neutron-scattering fixed-window scans
(FWSs), and quasielastic neutron scattering (QENS) measurements. Structural
characterization showed that the mixed compounds exhibit disordered
(α) and ordered (β) phases for temperatures above and
below about 200–250 K, respectively, in agreement with their
monoalkali correspondents. Vibrational and dynamical properties are
strongly influenced by the cation environment; in particular, there
is a red shift in the band energies of the librational and bending
modes with increasing lattice size as a result of changes in the bond
lengths and force constants. Additionally, slightly broader spectral
features are observed in the case of the mixed compounds, indicating
the presence of structural disorder caused by the random distribution
of the alkali-metal cations within the lattice. FWS measurements upon
heating showed that there is a large increase in reorientational mobility
as the systems go through the order–disorder (β–α)
phase transition, and measurements upon cooling of the α-phase
revealed the known strong hysteresis for reversion back to the β-phase.
Interestingly, at a given temperature, among the different alkali
silanide compounds, the relative reorientational mobilities of the
SiH3– anions in the α- and β-phases
tended to decrease and increase, respectively, with increasing alkali-metal
mass. This dynamical result might provide some insights concerning
the enthalpy–entropy compensation effect previously observed
for these potentially promising hydrogen storage materials.