4.2 Quasi-elastic neutron scattering measurements
4.2.3 Conclusions
QENS experiments was performed on different ball milled LiBH4 samples and on a range of temperature including the crystal phase transition. The above results show that vibrations in the low energy transfer might play an important role in understanding the reorientational dynamics of the [BH4] units. Anharmonic effects increase dramatically as soon as the phase transition is approached [128,129]. In this work, a transition in the inelastic signal is found:
at 343 K, spectra show a clear inelastic peak at 9 meV, whereas, at 403 K, the inelastic dynamic merges in the quasi-elastic region, superimposing to the localized reorientation dy-namics.
Already at 373 K, the elastic signal is reduced, and at the same time the reorientation dynamics become more disordered. From an uni-axial C3 rotations at 343 K, to a tum-bling motion at 373 K, where all the H atoms are reorientating in time. Above the crystal phase transition temperature, an almost free rotation is approached with the C6−HT “orbit exchange” model. This was also confirmed from MD simulation [129], where reorientations occur when hydrogen atoms move, on average, with 120◦ rotational jumps around the C3 -axis, repositioning all four hydrogen atoms of a [BH4] unit.
Taking into account the underlying vibrations, it was possible to extract the mean square displacement of H atoms. With increasing ball milling time the fast vibrational dynamic is enhanced, resulting in an increased hydrogen mobility.
This general approach, considering a convolution of the rotational and vibrational motions, allowed a comprehensive analysis of quasi-elastic and inelastic signal.
Magnesium Borohydride
Contents
5.1 Sample characterization . . . . 89 5.2 Experimental . . . . 92 5.2.1 β−Mg(BH4)2 . . . . 92 5.2.1.1 Data analysis. . . . 95
Data Analysis at λ=6Å . . . . 97 Data Analysis at λ=2.5Å . . . 100 5.2.1.2 Discussion . . . 105 5.2.1.3 Conclusions. . . 108 5.2.2 α−Mg(BH4)2 . . . 108 5.2.2.1 Data analysis. . . 111
Data analysis atλ1=6Å . . . 111 Data analysis atλ2=2.5Å . . . 115 5.2.2.2 Discussion . . . 124 5.2.2.3 Conclusion . . . 125 5.3 General conclusions on Mg(BH4)2 . . . 126
In this chapter, quasi-elastic neutron scattering experiments at different time scales and on α-Mg(BH4)2 and on β-Mg(BH4)2 are described, in order to elucidate the dynamics on both crystal species.
5.1 Sample characterization
Both samples were prepared as described in [68] at the Karlsruher Institut für Technologie, Karlsruhe, Germany.
Natural Boron contains roughly 80 % of 10B, which is a great absorber for neutrons. In order to avoid absorption of neutrons inside the sample, isotope enriched11B was used. Af-ter the synthesis, X-ray diffraction (XRD), thermo-gravimetric (TG), differential scanning calorimetry (DSC) and mass spectroscopy (MS) measurements were performed in order to check the quality and crystal phases of the samples.
300 400 500 600 700 800
massloss[%] DSCsignal[arb.units]
- Mg(BH
Figure 5.1: TG (blue and left axis) and DSC (green and right axis) measurement onα-Mg(BH4)2.
Forα-Mg(BH4)2, 4.57 mg of sample were used in the measurements, whereas for theβ-phase 17.71 mg. All DSC, TG and MS measurements were performed at the same time under inert (He) gas flow at a heating rate of 5 K min−1, and results are shown in figs.5.1, 5.2 and 5.3.
The α-phase shows an endothermic peak around 470 K (fig. 5.1), where the material trans-forms into the β-phase: the peak is quite narrow, indicating that the transformation is completed in few Kelvins. The crystal phase transition is not accompanied by any signifi-cant hydrogen release. Both samples decompose and start to release hydrogen above 550 K, as it can be observed from the TG curve, where the major release of hydrogen takes place between 550 and 600 K. This is also confirmed by MS measurements (fig. 5.3), where the three peaks indicate the intensity of H2 signal. The second endothermic peak corresponds to the first decomposition step of Mg(BH4)2, where ca. 10% wt.H2 is released.
Above 670 K, a successive H2 release is confirmed by the third DSC peak: this is interpreted as the decomposition of MgH2 into Mg, as described in literature [72]. The total amount of hydrogen released is in good agreement with theoretical values of 14.9 wt.% H2.
DSC measurements on β-Mg(BH4)2 show a different behaviour compared to the α-phase (fig. 5.2): the absence of the peak at 470 K confirms that the material is already in the β-phase. The MS measurements of H2 shows at least four intensity peaks (fig. 5.3): the first three in the region between 550 K and 600 K, where the TG measurements show the most amount of hydrogen release, and the fourth around 700 K, where a successive H2 release takes place.
As can be observed, hydrogen release does not happen in a single step decomposition.
In-300 400 500 600 700 800 -16
-14 -12 -10 -8 -6 -4 -2 0
massloss[%]
T [K]
Mg(BH 4
) 2
DSCsignal[arb.units]
Figure 5.2: TG (green and right axis) and DSC (orange and left axis) measurement onβ-Mg(BH4)2.
300 400 500 600 700 800
-16 -14 -12 -10 -8 -6 -4 -2 0
massloss[%]
T [K]
H
2MSsignal[arb.units] Mg(BH
4 )
2
Figure 5.3: TG (blue and left axis) and MS (red and right axis) measurement onβ-Mg(BH4)2.
stead, a multi-reaction decomposition starts at 550 K and continues till 700 K, and interme-diate compounds are possibly formed [72].
In this work, neutron scattering experiments were done up to 530 K, thus below the main hydrogen release.
5.2 Experimental
Quasi-elastic neutron scattering experiments were performed on α-Mg(BH4)2 and on β-Mg(BH4)2.
Approximatively 200 mg of Mg(11BH4)2 were mounted in an aluminium flat cell, yielding to calculated neutron transmission of ∼ 90%. The large transmission allows to minimize multiple scattering effects, although they can not be excluded at all.
In the experiments, two different wavelengths of incoming neutrons were selected: λ1 = 2.5Å yielding an energy resolution of450µeV(FWHM) of the elastic line and an accessible elastic momentum transfer Q from 0.5 to4.2Å−1, the second, λ2 = 6Å, with an energy resolution of 48µeV(FWHM) and a momentum transfer from 0.2 to 2Å−1.
Both samples were measured in a range of temperatures from 11 K to 500 K: at 11 K, the measured width of the elastic peak at zero energy transfer reflects the instrumental resolu-tion funcresolu-tion. At higher temperatures, the quasi-elastic and inelastic signals were measured, and, at 500 K, the transition from α- to β-phase occurs.
The raw time-of-flight data were converted to the scattering function S(Q, ω) following the procedure described in section 3.6. During the analysis, detector angles of the aluminium Bragg peaks were excluded.
In order to elucidate the dynamics in both samples, the data analysis will be divided according to the sample. The first measurements were done on β-Mg(BH4)2, in order to ex-tend work previously done [99]. Subsequently, measurements were performed onα-Mg(BH4)2 with the aim to shed some light on the dynamics of different crystal phases.