Nuclear Magnetic Resonance Study of Anion and Cation Dynamics in CsSiH3

In order to study the dynamical properties of cesium silanide CsSiH3, we have measured the 1H and 133Cs nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in this compound over the temperature range of 5-354 K. The results of the 1H NMR measurements indicate that [SiH3] anions in CsSiH3 retain unusually high reorientational mobility down to low temperatures. In particular, a significant narrowing of the proton NMR spectrum due to [SiH3] reorientations is observed in the range ~10-14 K. The order-disorder (..beta.. --> ..alpha..) phase transition accompanied by the strong acceleration of [SiH3] reorientations and by the change in the reorientational mechanism is observed above 200 K; according to the 1H spin-lattice relaxation data, this transition appears to be gradual. For the high-temperature disordered a-phase, the activation energy for [SiH3] reorientations is found to be 41(4) meV. The 133Cs NMR results are consistent with the onset of diffusive motion of Cs+ cations at the frequency scale of ~104 s-1 above 300 K.