Fast anion reorientations in ammine yttrium borohydride compounds Y(BH4 )3 ·xNH3 (x = 3 and 7): a nuclear magnetic resonance study

Abstract 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.