Hydrogen dynamics in β-Mg(BH₄)₂ on the picosecond timescale

Abstract

A quasielastic neutron scattering study on β-Mg(BH4)2 has been performed to investigate the hydrogen dynamics on the picosecond time-scale. Both vibrational and rotational motions of the [BH4](-) tetrahedra contribute to the signal at low energy transfers. A comprehensive analysis of the elastic and quasielastic incoherent structure factors allowed the separation of different parts. Below 200 K, vibrations and rotations (around the C2 or C3 symmetry axis of the [BH4](-) tetrahedra) are well separated. Above that temperature, a transition is observed in the vibrational part, and the spectral weight is shifted towards the quasielastic region. The dynamic transition is not accompanied by any structural phase change but we suggest that it is correlated with the anomalous thermal expansion that has been reported for β-Mg(BH4)2 [Filinchuk, et al., Chem. Mater., 2009, 21, 925].

A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth borohydrides LiBH4 and Mg(BH4)2 and the mixture LiBH4 + Mg(BH4)2

“…Measurements were performed at l 1 and l 3 , in the temperature range between 11 K and 500 K. Each temperature was measured for 3 hours, with additional 30 minutes of equilibration time. The data obtained for b-Mg(BH 4 ) 2 have been reported previously 45 and the results will be discussed in conjunction with the data presented here.…”

“…With the phase transition to the b-phase, both the quasielastic and inelastic signals undergo a distinct transformation and the data measured at 500 K were consistent with the findings published for b-Mg(BH 4 ) 2 . 45 The low lying excitations in a-Mg(BH 4 ) 2 have not been reported in previous studies of hydrogen dynamics in the a-phase. 48,49 The data analysis was conducted as described in the previous section.…”

“…It has been shown previously 45 that in b-Mg(BH 4 ) 2 , low energy vibrational modes inhibit the separation of rotation and vibration parts of the QENS spectra. Therefore, for the analysis, the convolution of S rot and S vib had to be considered in full.…”

“…In the b-modification, hu 2 i follows approximately the same relationship below 200 K, but between 200 and 300 K, a jump like increase is observed. In the same temperature region, the jump rotation processes in b-Mg(BH 4 ) 2 become fully activated 45 whereas the rotation modes in a-Mg(BH 4 ) 2 are still significantly hindered even at 400 K. Incidentally, the anomalous lattice expansion observed in b-Mg(BH 4 ) 2 shows a change from contraction to the expansion of the a-lattice parameter around 200 K and the opposite for the b-axis lattice parameter, indicating a link between the local rotational dynamics, lattice dynamics and structure.…”

“…In comparison, the metastable b-phase exhibited slightly faster motions and a larger temperature dependence at temperatures below 500 K. Similarly to the a-phase, two rotational motions were identified and the values G i,b are included in Table 1 (values taken from ref. 45). Table 2 shows the mean resident times of H atoms between two successive jumps around the C 2 rotation axis, together with the b-Mg(BH 4 ) 2 data in the same temperature range.…”