Abstract
Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH3, where M = K, Rb, Cs, Ki(0.5)Rb(0.5), 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 (cc) and ordered (beta) phases for temperatures above and below about 200250 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 (beta-alpha) phase transition, and measurements upon cooling of the alpha-phase revealed the known strong hysteresis for reversion back to the beta-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH3- anions in the alpha- and, beta-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.