Pressure-induced structural changes in Methylamine borane and dimethylamine borane

Article abstract of DOI:10.1016/j.jallcom.2017.06.174

Methylamine borane and dimethylamine borane have been studied under compression to 3 GPa using Raman spectroscopy and synchrotron powder X-ray diffraction. Both undergo reversible pressure-induced structural changes in this pressure range. The structural changes in the case of methylamine borane may be indicative of a second-order phase transition, taking place between ca. 0.8–1.2 GPa, which does not result in a change of space-group symmetry. In the case of dimethylamine borane, however, a reversible, reconstructive phase transition (monoclinic → orthorhombic) occurs below 0.7 GPa. This new high-pressure phase was successfully indexed, with a possible space group assignment of Pccn or Pbcn.

Full text of DOI:10.1016/j.jallcom.2017.06.174

Journal of Alloys and Compounds 722 (2017) 953e961
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Pressure-induced structural changes in Methylamine borane and
dimethylamine borane
a
,
,
Petra A. Szilagyi *, Steven Hunter b c, Carole A. Morrison b, Chiu C. Tang d,
ꢀ
ꢀ
,
c
Colin R. Pulham b
a University of Greenwich, Medway Campus, Chatham, ME4 4TB, UK
b School of Chemistry and EaStCHEM Research School, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh, EH9 3JJ, UK
c Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, UK
d Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Methylamine borane and dimethylamine borane have been studied under compression to 3 GPa using
Raman spectroscopy and synchrotron powder X-ray diffraction. Both undergo reversible pressure-
induced structural changes in this pressure range. The structural changes in the case of methylamine
borane may be indicative of a second-order phase transition, taking place between ca. 0.8e1.2 GPa,
which does not result in a change of space-group symmetry. In the case of dimethylamine borane,
however, a reversible, reconstructive phase transition (monoclinic / orthorhombic) occurs below
0.7 GPa. This new high-pressure phase was successfully indexed, with a possible space group assignment
of Pccn or Pbcn.
Received 26 April 2017
Received in revised form
14 June 2017
Accepted 15 June 2017
Available online 17 June 2017
The paper has been presented in Sympo-
sium O: Functional metal hydrides, at the E-
MRS Fall Meeting and Exhibit, Warsaw,
September, 2016.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Hydrogen storage
Phase transition
High pressure
Synchrotron X-ray powder diffraction
1. Introduction
respect to decomposition. Experimental efforts are therefore
focussed on catalytic or chemical-doping methods to destabilise
With increasing demands on fossil fuels, many industrialised
nations are turning towards alternative sources of energy. The
“hydrogen economy”, in which hydrogen is used as a “green”
feedstock in fuel cells to power motor vehicles, homes, etc. has been
highlighted as a potential solution to the energy problem. Chal-
lenges to be faced before this becomes a reality include the
development of sustainable methods of hydrogen production that
do not involve fossil fuels, and the safe and reversible storage of
hydrogen [1e4]. Several promising areas have been identified,
especially in the use of solid hydrogen-containing materials.
Although these materials contain substantial amounts of hydrogen
by weight percent, release of hydrogen gas is often difficult to
achieve on account of the high stability of these materials with
these hydrides using a range of empirical approaches [5e7]. An
important step in developing an understanding of the chemical
properties of such materials is the study of structure, with a
particular focus on the structural response as a function of both
temperature and pressure. In addition to developing an under-
standing of the relationship between structure and the potential for
hydrogen-storage, variable temperature and pressure studies may
also result in the formation of new phases with enhanced proper-
ties that may be recovered in bulk quantities at ambient conditions
[8,9].
Experimental mapping of polymorphism for light hydrides us-
ing X-rays is not a trivial task due to the presence of weakly scat-
tering hydrogen atoms. Furthermore, many of these materials are
polycrystalline and highly hygroscopic, which makes obtaining
good quality powder diffraction data challenging. Using powder X-
ray diffraction to identify and solve polymorphism of light hydrides
under pressure is further complicated by the possibility of multiple
* Corresponding author.
E-mail address: P.A.Szilagyi@greenwich.ac.uk (P.A. Szilagyi).
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0925-8388/© 2017 Elsevier B.V. All rights reserved.

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fits to the resulting pattern, which makes the use of complemen-
tary techniques indispensable. Neutron powder diffraction studies
are also hindered by strong absorption from the 20% abundance of
10B nuclei and incoherent scattering from 1H, thereby requiring the
synthesis of both 11B and 2H isotope-enriched samples [10]. This
can be both time-consuming and expensive. On the other hand,
Raman spectroscopy can help reveal crucial structural information
at the molecular level [11]. Theoretical modelling also yields
structural information, but it may also prove to be rather chal-
lenging as seen from the apparent disagreement between the
observed and theoretically predicted structures for LiBH4 [12e14]
and Mg(BH4)2 [15e18].
In this work, we present a study combining intense synchrotron
powder diffraction and Raman spectroscopy, aimed at exploring
the structural properties of methylamine borane and dimethyl-
amine borane. First-principles simulations were performed to aid
the assignment of the Raman spectra. Ammonia borane has been
highlighted as a promising material for chemical hydrogen-storage
applications [19e23]. Containing three protic NeH and three
hydridic BeH bonds and having a low molecular weight, it has the
potential to meet the rigorous gravimetric and volumetric
hydrogen-storage capacity requirements for mobile applications. It
has therefore been extensively studied. It is known that thermal
decomposition results in the release of hydrogen in several steps
between 100 and 500 ꢀC [24]. Infusion of NH3BH3 in mesoporous
silica, carbon cryogel, and metal-organic frameworks was found to
increase the dehydrogenation kinetics and also to suppress bor-
azine formation (a toxic by-product) [25e29]. Dissociation of
NH3BH3 is also reported to be activated by the addition of cation
exchange resins, zeolites and ionic liquids [30e32].
2. Experimental
All chemicals were purchased from Sigma Aldrich and were
used without further purification before syntheses.
2.1. Synthesis of methylamine borane
0.4266 g (11 mmol) sodium tetrahydridoborate was mixed with
0.8381 g (12 mmol) methylammonium hydrochloride in 20 cm3
anhydrous THF under N2 atmosphere in a reaction vessel equipped
with a gas bubbler, similar to the procedure explained by Bowden
et al. [51]. Formation of H2 gas was observed immediately after the
addition of the reactants into the vessel. The reaction mixture was
stirred under constant N2-flow at 45 ꢀC for 2 h, the heating and gas-
flow were then stopped and the mixture was left stirring overnight
at ambient temperature. The resultant slurry was then filtered off
and washed with 3 cm3 anhydrous THF under N2-atmosphere. The
filtered solution was then evaporated under vacuum and white,
polycrystalline methylamine borane was obtained, which was
characterised by Raman spectroscopy.
2.2. Synthesis of dimethylamine borane
Dimethylamine borane was prepared using the same protocol as
for the methylamine borane by the use of 0.4744 g (12 mmol) so-
dium tetrahydridoborate and 1.0093
lammonium hydrochloride. The resulting white polycrystalline
material was also characterised by Raman spectroscopy.
g (12 mmol) dimethy-
Because of the presence of BeH/HeN bonds in solid-state
ammonia borane, its behaviour under pressure has also been
investigated with a variety of methods, such as Raman spectros-
copy [33e36], X-ray [37e39] and neutron diffraction [39e41], and
computational methods [42e46]. It should be noted that owing to
the intrinsic softness of ammonia borane most studies did not use
any pressure-transmitting medium except for that by Custalcean
et al. [47], in which hydrostatic conditions were thus ensured.
Raman studies have suggested phase transitions at 2.0, 5.0 and
12.0 GPa, and combined data from neutron and X-ray diffraction
have found a body-centred tetragonal I4mm ambient phase that
undergoes a transition into a Cmc21 orthorhombic phase at ca.
1.2 GPa, which subsequently transforms into a P1 triclinic phase at
approximately 8.2 GPa [37,40,48e50].
2.3. High-pressure measurements: general procedures
High-pressure Raman spectroscopy and X-ray diffraction ex-
periments were performed using a Merrill-Bassett diamond-anvil
cell (40ꢀ half-opening angle), equipped with 600
mm culets and a
tungsten gasket with a 300
mm hole [70]. The sample and a chip of
ruby (as a pressure calibrant) were loaded into the diamond-anvil
cell with Fluorinert (FC-77) as a pressure-transmitting medium.
This was chosen as the samples were found to be soluble in other
media such as mixed pentanes, glycerin and methanol-ethanol
mixtures. Furthermore, they decompose in methanolic solutions.
The induced pressure was determined by monitoring the ruby
crystal R1 fluorescence wavelength by fitting Lorentzian lineshapes
to the R1 line: the resulting uncertainty is 0.04 GPa [71].
N-substituted ammonia borane derivatives, such as methyl-
amine borane and dimethylamine borane [CH3NH2BH3 and
(CH3)2NHBH3] have theoretical hydrogen capacities of 17.9 and
17.0 wt%, respectively, of which 9.0 and 6.8 wt% hydrogen is
experimentally available [51e53]. Furthermore, Sun and co-
workers have pointed out that N-methyl substitution of ammonia
borane enhances the reversibility of the system and prevents the
formation of diborane and ammonia e two major contaminants in
the case of ammonia borane [54]. Dimethylamine borane is also of
great interest as it is known to undergo dehydrogenation in the
presence of transition-metal catalysts to give hydrogen gas and the
cyclic (dimethylamino)borane dimer, (Me2NBH2)2, as final products
[55e67].
2.4. Raman Spectroscopy
Raman measurements were conducted at ambient temperature
on the powdered samples. Spectra were recorded on a LabRam
instrument equipped with a 50 mW He-Ne laser of wavelength
632.8 nm.
2.5. Synchrotron X-ray powder diffraction (SXPD) data collection
Diffraction data was collected on beamline I11 at the Diamond
Light Source [72] using
a monochromatic X-ray beam of
Despite the high hydrogen content, the presence of non-
classical BeH/HeN bonds in solid methylamine and dimethyl-
amine borane [68], and the effort invested in the study of ammonia
borane, there has been only one Raman-spectroscopy study pub-
lished on the behaviour of dimethylamine borane under pressure
using [69]. In this paper, we report the structural changes occurring
in the methylamine borane and N,N-dimethylamine borane sys-
tems under quasi-hydrostatic pressure up to 3 GPa.
l
¼ 0.48512 Å. A mar345 image plate was used to collect angle-
dispersive diffraction patterns. The sample-to-detector distance
and the wavelength were calibrated using a CeO2 powder standard.
The pressure was measured ex-situ using ruby fluorescence lines
from small embedded crystals in the sample before collecting each
diffraction pattern. The data was integrated using the program
Fit2D [73] with masks to avoid integration of regions of the detector
shaded by the body of the pressure cell.

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2.6. Structure analysis, reduction and refinement
BeN-stretching mode with isotope splitting are at somewhat lower
wavenumbers in the case of methylamine borane (738 and
753 cmꢁ1) than for ammonia borane [80] (784 and 800 cmꢁ1). A full
listing of assigned vibrational modes is available in Table A1 of the
Supplementary Information.
Space groups and unit cells of the materials under study were
determined from the powder patterns by indexing software Crys-
fire [74], verified by LeBail refinement in GSAS [75] analytical
software. Single-crystal structures as determined by Bowden and
co-workers [51] for the methylamine borane and by Aldridge et al.
[68] for the dimethylamine borane were used as models in FOX
1.7.7.7-R1013 [76]. This data was then re-loaded into GSAS and
Rietveld refinements were carried out on the heavy atoms, first
with constraints applied on the bond lengths and angles which
were gradually lifted. Hydrogen atoms were then added in the
structure, in accordance with the molecular geometry calculation
in WinCrystals 2000 [77]. The resulting crystallography informa-
tion file was re-loaded into GSAS and the final Rietveld refinement
carried out by first handling the molecular structure as a rigid body
(in effect fixing the hydrogen atom positions). All constraints were
then gradually released as the least-squares fitting procedure
converged on a minimum structure.
On increasing the pressure, spectral changes, indicative of
structural variation in the condensed phase of methylamine borane
were observed (see Fig. 2a and c). In general, all modes blue-shifted
(hardened) with respect to external pressure, but those involved
with the intermolecular dihydrogen bonds (the NeH stretching and
deformation modes) red-shifted (softened). The same pressure-
induced phenomenon of mode softening was observed for
ammonia borane and it was interpreted as being indicative of the
strengthening of the dihydrogen bonds in the solid state, which
consequently results in the weakening of the NeH bonds [34,47]. In
addition, for the NeH scissoring mode we note that the corre-
sponding band does not split into two at higher pressures, as
observed for ammonia borane.
The evolution of Raman modes as a function of applied pressure
shows some changes taking place in the 1e1.5 GPa region. Close to
linear plots were obtained for vn=vp with slight divergence at
2.7. Computational methods
pressures above 1.5 GPa for modes in the region of
3130e3350 cmꢁ1 and 1430-1620 cmꢁ1 (see Fig. 2b and d). This
discrepancy between the Raman peak maxima at the low- and
high-pressure regions may be indicative of pressure-induced
structural changes.
Structure optimisations were performed using density func-
tional theory (DFT) and the plane-wave pseudopotential method as
implemented in CASTEP version 5.5 [78]. Following geometry
optimisation, phonon frequencies were calculated using the finite
displacement method, as executed within the CASTEP code. Visu-
alisation of individual wave-vectors, which allowed mode assign-
ment, were performed using the Jmol software package [79].
Structural Characterisation: Synchrotron X-ray Diffraction. The
structure of methylamine borane was first determined by M. E.
Bowden and co-workers at 113 K using single-crystal diffraction
[78]. The authors found that the compound crystallises in the
orthorhombic Pnma space group and also identified the presence of
intermolecular BeH/HeN dihydrogen bonds (2.218 Å). Visual-
isation of the molecular packing in Mercury [81] reveals interchain
BeH/HeC interactions of 2.764 Å and CeH/HeC interactions of
2.800 Å [51]. Single-crystal data (150 K) were collected on the
compound by S. Aldridge et al. (Fig. 3). They found that molecules of
the methylamine borane adduct are linked in ribbons running
along the b-axis and within these ribbons pairs of molecules are
aligned in an antiparallel fashion, with the BeN bonds along the c-
axis. Visualisation of their single-crystal data shows that each pair
of molecules is linked by two BeH/HeN dihydrogen bonds of
2.130 Å and that the ribbons are linked in a layer through
BeH/HeN contacts of 2.504 Å, and BeH/HeC interactions of
2.763 and 2.979 Å which hold the layers together [68].
3. Results and discussion
3.1. Methylamine borane
Raman Spectroscopy. The Raman spectrum of methylamine
borane at ambient pressure (Fig. 1) shares many features with that
of solid ammonia borane [80]. To aid interpretation, the spectrum
was compared with the computationally calculated eigenvalues,
where agreement was observed for all modes to within 4%. The
major difference between the spectrum of methylamine borane
and that of ammonia borane arises from the presence of the methyl
group in the former. In addition, the characteristic bands for the
SXPD data were collected on polycrystalline methylamine
borane at ambient temperature, as a function of pressure up to
3.0 GPa. The resulting patterns (Fig. 4) indicate that peaks shift to
higher 2q values with increasing pressures, as expected. As no
peaks appear or disappear upon changing pressure, we can safely
conclude that there is no pressure-induced first-order phase tran-
sition taking place in the condensed phase of methylamine borane
at ambient temperature conditions, up to 3.0 GPa.
In order to verify whether second-order phase transitions have
taken place in methylamine borane system each of the diffractions
patterns was fitted to model structures. Lattice parameters for all
the patterns have been successfully refined using the orthorhombic
(Pnma) structure of the ambient-pressure form as the starting
structural model (see Table 1).
Compression plots of the lattice parameters and the cell volume
(Fig. 5) show some changes in their gradients between ca. 0.8 and
1.5 GPa. This suggests that, while there is no reconstructive first-
order phase transition occurring in the system up to 3 GPa,
pressure-induced structural changes or even a second-order phase
transition may have occurred over this pressure range.
Fig. 1. Ambient-temperature Raman spectrum of CH3NH2BH3.
Taking a closer look at the compression plots of the lattice

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Fig. 2. Changes in the Raman spectra for methylamine borane with respect to pressure (a) 3130-3350 cmꢁ1 region : NH2 asymmetric stretch, ꢂ NH2 symmetric stretch, and - BH3
symmetric stretch þ NH2eBH3eCH3 wag, (c) 1430-1620 cmꢁ1 region
NH2 scissor, +CH3 scissoring, and
CH3 twist, (b) and (d) pressure range (in GPa): a:0; b:0.04; c:0.4; d:0.6,
;
A
e:0.8, f:1.1, g:1.2; h:1.4; i:1.6; j:1.8; k:2.1; l:2.2; m:2.4; n:3.0.
Fig. 4. Powder X-ray patterns of CH3NH2BH3 at pressures of (a) 0.7 GPa, (b) 1.5 GPa and
(c) 3.0 GPa.
Fig. 3. Crystal packing of CH3NH2BH3 at ambient pressure highlighting the BeH/HeN
contacts.
direction of the NeH/HeB dihydrogen bonds, as determined by
Aldridge and co-workers [68]. It is also worth noting that the NeH
parameters (Fig. 5a), we observe that the low and high-pressure
trends differ the most along the b-axis. This coincides with the

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Table 1
Refined structural parameters for the CH3NHBH3 as a function of pressure.
Pressure, GPa
a, Å
b, Å
c, Å
Vcell, Å3
0 [51]
0 [68]
0
11.099
11.135
6.584
6.558
4.919
4.919
359.460
359.204
11.164 (2)
10.972 (1)
10.718 (3)
10.664 (3)
10.516 (7)
10.489 (2)
10.361 (4)
10.285 (9)
10.128 (9)
6.7384 (7)
6.6681 (2)
6.584 (2)
6.558 (1)
6.555 (3)
6.4963 (9)
6.439 (2)
6.390 (4)
6.343 (4)
4.9708 (7)
4.9247 (4)
4.871 (1)
4.854 (1)
4.817 (2)
4.8036 (6)
4.771 (1)
4.743 (4)
4.698 (3)
373.927 (2)
360.317 (1)
343.743 (3)
339.467 (3)
332.035 (7)
327.319 (2)
318.302 (4)
311.715 (9)
301.783 (9)
0.4
0.7
0.8
1.2
1.5
1.8
2.3
3
and BeH stretching and deformation modes were found to undergo
the most significant changes in the Raman spectra above 1 GPa. In
addition, up to 3.0 GPa, the a-axis was found to be around 1.65
times more compressible than both the b and c axes. The difference
in compressibility along the a-axis can be explained by close in-
spection of the crystal packing of methylamine borane (Fig. 3) [68].
The nearest intermolecular heavy-atom distance occurring along
the direction of the a-axis in the ambient pressure structure is dC …
Fig. 6. Raman spectrum of (CH3)2NHBH3 recorded under ambient conditions.
3.2. Dimethylamine borane
¼ 4.239 Å, while for the b- and c-axes they are substantially
N
shorter: dB … N ¼ 3.509 Å and dB … N ¼ 3.448 Å, respectively [68].
Compression plots of the cell parameters thus suggest that there
may be pressure-induced structural changes in the 0.8e1.2 GPa
range. The compression plot of the cell volume (Fig. 5b) also sug-
gests that the compressibility of the unit cell differs in the low-
pressure and high-pressure regions.
Raman Spectroscopy. The Raman spectrum recorded of dime-
thylamine borane at ambient temperature and pressure is pre-
sented in Fig. 6. First-principles calculations were again used to aid
spectral interpretation, with all calculated modes found to be in
agreement with experimental modes to within 5%. The character-
istic Raman shifts for the BeN stretching modes (712 and
723 cmꢁ1) are somewhat lower than what was published previ-
ously [69] and they are significantly lower than those observed for
methylamine borane (738 and 753 cmꢁ1, this work) and ammonia
borane (784 and 800 cmꢁ1 [80]) due to the different BeN bond
lengths 1.596 Å [68] for the dimethylamine, 1.594 Å [51] for the
methylamine and 1.58 Å for ammonia borane [83]. For mode as-
signments refer to Table A2 in the Supplementary Information.
Similar to the cases of ammonia borane and methylamine
borane, the compression of dimethylamine borane resulted in
shifts to higher wavenumber of most bands, except for the NeH
stretching modes that were red-shifted upon the application of
pressure (Fig. 7b), indicative of the strengthening of intermolecular
It should be noted that at these pressures the pressure-
transmitting medium used should no longer be hydrostatic (Fluo-
rinert FC77 is hydrostatic up to only 1.0 GPa) [82], which implies
that its use may have introduced pressure-related stress gradients
within the diamond-anvil cell. However, no signs of broadening of
the diffraction peaks or broadening of the ruby fluorescence line
were observed, suggesting that, at least for these low pressures, the
intrinsic softness of the sample retains hydrostatic conditions.
The Raman spectroscopic and SXPD data all suggest the exis-
tence of pressure-induced structural changes in the methylamine
borane system at ambient temperature around 1.2 GPa. These
changes may also be indicative of a second-order phase transition.
Fig. 5. (a) Compression plot of cell parameters for CH3NH2BH3 as a function of pressure: black square e a/a0, red circle: b/b0 and blue triangle e c/c0. Note data points are connected
by straight lines as a visual guide. (b) Variation of the unit cell volume of CH3NH2BH3 as a function of pressure. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)

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Fig. 7. From left to right: in the 2150-2600 cmꢁ1 and in the 3150-3250 cmꢁ1 regions at a: ambient, b: 0.7 and c: 1.0 GPa pressure.
contacts [33e36,47].
However, in contrast with the case of methylamine borane,
increasing the pressure to 0.7 GPa results in apparent changes in
the Raman spectrum of dimethylamine borane (see Fig. 7a). For
example the peak at ca. 2298 cmꢁ1 (a BeH stretching vibration)
collapses at 0.7 GPa, together with the peak at 2345 cmꢁ1, which is
assigned to a complex combination of the CH3 deformation, NH-
wagging, CeN stretching and BH3 and CH3 deformation modes.
These differences may indicate that the material undergoes a phase
transition below 0.7 GPa. However, this observation has been pre-
viously explained by different pressure dependencies of the Fermi
resonance between fundamental/overtone pairs and thus has not
been attributed to a phase transition [69]. In addition, the NeH and
BeH vibration modes display a significant shift upon pressure in-
crease to 0.7 GPa. It is interesting to note that in the case of
ammonia borane a BeN bond shifted by 1.3% for every GPa applied
[47], whereas for dimethylamine borane a staggering 2.7% shift was
observed. In addition, the BeN stretching mode remained in the
Raman spectrum as one doublet, displaying the 10B-11B isotope
splitting. Given the extreme sensitivity of this band frequency to
the BeN bond length [34,37,47], we can therefore safely assume
that there is only one dimethylamine conformation present when
the sample is compressed.
Fig. 9. Powder X-ray diffraction pattern of (CH3)2NHBH3 at a: ambient; b: 0.8 GPa; c:
1.0 GPa; d: 3.2 GPa and at e: ambient pressure after decompression.
Structural Characterisation: SXPD. The structure of dimethyl-
amine borane was first determined by S. Aldridge and co-workers
at 150 K using single-crystal diffraction (Fig. 8) [68]. They found
that the compound crystallises in the monoclinic P21/c space group,
which is built up from chains of the adduct molecules running
Fig. 10. X-ray powder patterns of the (CH3)2NHBH3 under 1.0 GPa as refined in the a:
Pccn and b: Pbcn space groups. Black line: observed pattern, crosses: calculated
pattern, green tick marks: modelled peaks, pink line: difference of the observed and
calculated patterns. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Fig. 8. Crystal packing of (CH3)2NHBH3 at ambient pressure.

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Table 2
Refined lattice parameters for the (CH3)2NHBH3 at ambient pressure and 1 GPa, using the Pccn and Pbcn orthorhombic models.
Vcell, Å3
Z
c2
ꢀ
Space group
a, Å
b, Å
c, Å
b,
150 K, ambient Single crystal [68]
Room temperature, 1.0 GPa
Room temperature, 1.0 GPa
P21/c
Pccn
Pbcn
7.0452
14.98 (5)
14.98 (6)
5.8368
5.83 (2)
5.83 (3)
12.2335
9.96 (4)
9.96 (6)
104.65
90
90
486.708
870
870
4
8
8
R ¼ 0.042
a
wRp ¼ 0.2201
a
wRp ¼ 0.2207
a
Background corrected.
along the b-axis. They demonstrated that H/H bonding links the
molecules head-to-tail, shown as a bifurcated BH2/HN interaction,
with H/H distance of 2.083 and 2.202 Å [68]. The chains are
relatively closely packed, with the interchain H/H linking dis-
tances being 2.562 and 2.684 Å for the shortest BH/HC and
CH/HC distances, respectively [68].
SXPD up to 3 GPa, at room temperature. The study is also supported
by a DFT study, which permitted the assignment of the Raman
spectra. Up to 0.6 GPa the crystal structures remain in their ambient
pressure phase. Here the molecules pack so that ribbons of adducts
held together via BH/HN bonds run along the b-axis of the unit
cells, which are linked by weaker H/H interactions between them,
directed in the ac planes.
In order to determine the high-pressure phase of dimethyl-
amine borane, synchrotron powder X-ray data were collected on
polycrystalline dimethylamine borane at ambient temperature, as a
function of pressure up to 3.2 GPa. On raising the pressure to
0.8 GPa (see Fig. 9a and b) a dramatic change was observed in the
pattern indicating the formation of a new phase. This persisted up
to 3.2 GPa although with broadening of peaks, but reverted to the
ambient-pressure phase on decompression. Inspection of the two-
dimensional diffraction images showed that the degree of texture
(preferred orientation) of the sample was substantially reduced on
passing through the phase transition, indicating that a recon-
structive transition had taken place. This is also reflected in the
observation from Fig. 9 that the two patterns recorded at ambient
pressure show some peaks with very different intensities. Dime-
thylamine borane is very hygroscopic and decomposes in air, thus
in order to minimise this effect, the sample was only lightly ground
before loading into the diamond-anvil cell. However, as a conse-
quence of this, powder averaging for the first ambient-pressure
pattern was poor. This resulted in anomalous intensities for some
Bragg peaks.
Between 0.8 and 1.2 GPa, methylamine borane undergoes
reversible pressure-induced structural changes, with a possibility
of the existence of a second-order phase transition in the same
space group. Both, our Raman and SXPD data support the
strengthening of the NeH/HeB dihydrogen bonds upon
compression and in the 0.8e1.2 GPa pressure range in particular.
At 0.7 GPa, dimethylamine borane undergoes a reversible,
reconstructive phase transition from the monoclinic P21/c space
group. Powder X-ray diffraction allowed indexing of the resultant
patterns to orthorhombic symmetry. However, despite our effort
and extensive use of complimentary methods such as SXPD, single-
crystal X-ray diffraction and Raman spectroscopy, we have been
unable to solve the high-pressure dimehtylamine-borane structure,
due to the insufficient quality SXPD data caused by preferred
orientation of the material. We can thus only conclude that a
reversible pressure-induced phase transition takes place as follows:
monoclinic (P21/c, a ¼ 7.0452, b ¼ 5.8368, c ¼ 12.2335,
b
¼ 104.648)
/ orthorhombic (a ¼ 14.98, b ¼ 5.83, c ¼ 9.96).
Finally, we would like to emphasise the necessity of using a
combination of various experimental techniques and theory to be
able to identify pressure-induced phase transitions in soft
materials.
It proved possible to index the pattern for the high-pressure
phase recorded at 1.0 GPa to an orthorhombic crystal system with
lattice parameters a ¼ 14.98, b ¼ 5.83, c ¼ 9.96 Å. Systematic ab-
sences suggested two possible space groups: Pccn or Pbcn (Fig. 10a
and b, Table 2). For the initial Rietveld refinements, atomic pa-
rameters were fixed.
Acknowledgments
From Table 2 we observe that the cell parameters, as well as the
errors and R-factors of the two high-pressure powder refinements,
are very similar.
ꢀ
ꢀ
P. A. Szilagyi thanks the AXA Research Fund for funding and the
Diamond Light Source for provision of synchrotron beamtime. S.H.
thanks the Scottish Funding Council (SPIRIT) and QinetiQ (in sup-
port of MOD research) for contribution towards a studentship. This
work made use of the facilities of HECToR, the UK's national high-
performance computing service, which is provided by UoE HPCx
Ltd at the University of Edinburgh, Cray Inc. and NAG Ltd, and
funded by the Office of Science and Technology through EPSRC's
High End Computing Programme. The authors would like to thank
D. I. A. Millar for the useful discussions.
In an attempt to resolve the identity of the space group, we
pursued high-pressure single-crystal diffraction experiments, since
this would provide a more accurate determination of the crystal-
lographic systematic absences. In addition, high-quality single-
crystal data may also provide some information that could
contribute to the determination of hydrogen atom positions. Suit-
able single crystals of the dimethylamine borane were thus ob-
tained and loaded into diamond-anvil cells using Fluorinert as the
pressure-transmitting medium. The pressure was then increased to
1.2 GPa, somewhat above the transition pressure. However, it was
found that the reconstructive phase transition caused the single
crystals to completely disintegrate and so no useful X-ray data
could be collected. Despite all our efforts, to date we have thus been
unable to resolve the ambiguity between the Pccn and Pbcn space
groups.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2017.06.174.
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