Vibrational properties of β-KSiH3and β-RbSiH3: a combined Raman and inelastic neutron scattering study

Abstract

The hydrogen storage materials ASiH3 (A = K and Rb) represent complex metal hydrides built from metal cations and pyramidal SiH3− ions. At room temperature, SiH3− moieties are randomly oriented because of dynamical disorder (α-modifications). At temperatures below 200 K, ASiH3 exist as ordered low-temperature (β) modifications. The vibrational properties of β-ASiH3 were characterized by a combination of Raman spectroscopy and inelastic neutron scattering. Internal modes of SiH3− are observed in the spectral range 1800–1900 cm−1 (stretching modes) and 890–1000 cm−1 (bending modes). External modes are observed below 500 cm−1. Specifically, SiH3− librations are between 300–450 cm−1 and 270–400 cm−1 for A = K and Rb, respectively, SiH3− translations are between 95 and 160 cm−1, K+ translations are in the range 60–100 cm−1 and Rb+ translations in the range 50–70 cm−1. The red-shift of libration modes for A = Rb is associated with a 15–30% reduction of the libration force constants of SiH3− ions in β-RbSiH3. This correlates with a lower temperature for the β–α order–disorder phase transition (278 vs 298 K). Libration modes become significantly anharmonic with increasing temperature but are maintained up to at least 200 K. The vibrational properties of ASiH3 compare well to those of alkali metal borohydrides ABH4 (A = Na–Cs). Copyright © 2016 John Wiley & Sons, Ltd.