ARTICLE IN PRESS
C.M. Brown et al. / Physica B 385–386 (2006) 266–268
267
upon confining AB to the mesoporous silica scaffold
remains to be answered. We have initiated a research
program to use quasielastic neutron scattering (QENS),
amongst other techniques, to attempt to answer this
fundamental question.
not be achieved). For this sample, we nominally should
include two Lorentzians in the fit, however, a proton NMR
study indicates that above 225 K all protons were moving
at essentially the same rate and it was hypothesized that the
molecule is performing whole-body reorientations [10].
Using incoherent neutron scattering we cannot add to this
hypothesis, but it may be possible to address the situation
with a completely deuterated sample or by labeling each
end of the molecule.
2
. Experimental details
1
Isotopically enriched B NH BH was prepared via a
1
3
3
multistep synthetic scheme beginning with the reaction of
1
Fig. 1 shows the results of our phenomenological fitting
a slice of data taken with DCS at 305 K. Good fits are
maintained as a function of both momentum transfer (Q)
and temperature. The data can be further reduced to
extract an elastic incoherent structure factor (EISF), which
is simply the ratio of elastic intensity to that of the elastic
plus the quasielastic intensity. A model that fits this EISF,
makes physical sense and is consistent with a previous
NMR study [10] but not an older one [11] is that of a
proton jumping around a three-fold axis. The EISF is
dependent on Q and the proton jump distance, R, as
EISF ¼ 1=3ð1 þ 2 sinðQRÞ=ðQRÞÞ. In this case, we found it
is necessary to have an extra Q-independent elastic
component that amounts to ꢀ10% of the scattering
intensity despite removal of the observable Bragg peaks
from the data reduction. In addition, this model imposes a
Q-independent width on the Lorentzian peak. While it is
possible to fit the current data with no constraints, at the
lower temperatures there are correlations between para-
meters that lead to an upturn in the width at higher Q.
Fixing the width to a mean width obtained in the mid-Q
range, where the Lorentzian intensity is greatest, does not
alter the quality of the fits at other Q values significantly.
Clearly, there are no 12-fold jumps or free rotations
occurring as suggested by Reynhardt and Hoon [11].
1
B enriched boric acid with anhydrous methanol. Tri-
methylborate was extracted and then added dropwise to
1
1
excess NaH and stirred at 220 1C. The resulting Na BH
4
1
was then purified. Finally, the Na BH was reacted with
1
4
ammonium carbonate at 40 1C, resulting in purified
1
1
NH BH [4] Sample purity was confirmed with
11
B
3
3
NMR, X-ray diffraction and infrared spectroscopy.
Neutron powder diffraction data taken on the BT1
diffractometer at the NCNR were analyzed within a
constrained Rietveld model (after Ref. [5]) and gave
satisfactory fit despite the high incoherent background.
Neutron vibrational spectroscopy performed on the FANS
spectrometer (also at NCNR) was entirely consistent with
previous spectra obtained on natural boron NH BH [6].
3
3
Labeled ammonia borane samples were absorbed onto a
mesoporous silica substrate by stirring a methanol solution
of ammonia borane over the silica substrate. Methanol was
removed with a rotary evaporator and the process was
repeated three times. The loaded sample was prepared with
equal masses of ammonia borane and mesoporous silica.
The Disk Chopper Spectrometer (DCS) [7] and the High
Flux Backscattering Spectrometer (HFBS) [8] at the NIST
Center for Neutron Research was used to study the bulk
1
1
and confined ammonia borane. Isotopically enriched
B
˚
samples were loaded into annular aluminum cells and
cooled in a closed cycle refrigerator.
The resulting proton jump distance of 1.89(1) A agrees
well with the BH distances from the Rietveld refinement
3
On DCS, data were collected at a series of temperatures
above the structural phase transition at 225 K to extract
activation energies for proton motion on 50 mg of the
NH BH sample and on a neat NH BH loaded MCM-41
3
3
3
3
2
˚
material (surface area of 890 m /g, pore diameter of 19 A).
An instrumental resolution of ꢀ0.26 meV is achieved using
˚
an incident wavelength of 3.6 A. On HFBS, data were
collected at several temperatures below 225 K to extract
information on proton motions at higher resolution
(
1.2 meV). By using both spectrometers, we can follow fast
proton motions that occur on a picosecond time scale on
DCS to slower motions on the nanosecond time scale on
HFBS. Data were reduced and fitted using standard
software available at the NCNR [9].
3
. Results and discussion
1
1
˚
ꢁ1
Fig. 1. DCS S(Q,w) data of NH
3
BH
3
at 305 K (circles) at Q ¼ 1:56 A
The reduced DCS data of the NH BH sample can be
3
3
with the total fit (solid line) and components (elastic: dotted; quasi-elastic
Lorentzian: dot-dashed; background: dashed). The inset shows the
extracted EISF at 250 and 305 K with a fit to a three-fold jump diffusion
model as detailed in the text.
analyzed in terms of a Gaussian elastic peak, a broad
background and a Lorentzian peak arising from quasie-
lastic scattering (an improved fit using 2 Lorentzians could
Brown, Craig M.
