Fast anion reorientations in ammine yttrium borohydride compounds Y(BH4)3·xNH3 (x = 3 and 7): a nuclear magnetic resonance study
摘要
Metal borohydrides are the ionic compounds which have attracted significant recent attention due to their potential applications as materials for hydrogen storage and solid electrolytes. The ammine borohydride compounds Y(BH4)3·xNH3 belong to the emerging class of materials, where the application-relevant properties can be tuned by introducing neutral NH3 ligands. To study the dynamical properties of Y(BH4)3·3NH3 and Y(BH4)3·7NH3, we have measured the 1H and 11B nuclear spin–lattice relaxation rates and nuclear magnetic resonance (NMR) spectra in these compounds over a wide temperature range (6–298 K). Our measurements have revealed that both compounds retain the unusually high reorientational mobility of BH4− anions down to low temperatures. In particular, for the fastest BH4 reorientational process, the corresponding H jump rate is of the order of 108 s−1 near 96 K in Y(BH4)3·3NH3 and near 33 K in Y(BH4)3·7NH3. The fast reorientational motion of BH4− anions is discussed in terms of the coordination environment of BH4 groups in Y(BH4)3·xNH3.