4
8
A. Fossdal et al. / Journal of Alloys and Compounds 387 (2005) 47–51
at 8 K are presented. For structural studies by neutron diffrac-
tion, use of the deuterated magnesium alanate, Mg(AlD4)2,
would have been preferable. This compound has, however,
not yet been synthesized, hence, the current investigations
were performed on the hydride, Mg(AlH4)2.
parameters were used and linear interpolation between man-
ually selected points (48 for PND at 8 K, 61 for PND at 111 K,
33 for PND at 295 K and 70 for PXD at 111 and 295 K) were
used for background modelling. In total, 29 parameters were
varied in the final refinement for the combined PND and PXD
295 K data. For Mg(AlH4)2: one scale factor, nine structural
parameters and eleven profile parameters, for NaCl: one scale
factor, one structural parameter and four profile parameters,
and in addition, two zero points. In the refinement of the
111 K data, one additional profile parameter was varied for
Mg(AlH4)2. In the 8 K refinement, 18 parameters were var-
ied. Mg(AlH4)2: one scale factor, ten structural parameters
and four profile parameters. NaCl: one scale factor and one
structural parameter. In addition, one zero point was varied.
2
. Experimental
Mg(AlH4)2 was synthesized via a metathesis reaction of
NaAlH4 and MgCl2 in diethyl ether, with subsequent purifi-
cation and solvent removal. The procedure is described in
detail elsewhere [14]. Due to the preparation route, NaCl is
present as a secondary phase in the resulting white powder.
NaCl was included in the Rietveld refinements (space group,
¯
˚
Fm3m, a = 5.6394(4) A at 295 K), and the content of NaCl
was found to be approximately 5 wt.%.
3. Results and discussion
Powder X-ray diffraction (PXD) data at 111 and 295 K
were collected at the Swiss–Norwegian beam line (station
BM01B) at the European Synchrotron Radiation Facility
The atomic coordinates published by Fichtner et al. [17]
were used as starting parameters in the Rietveld refinements.
(ESRF) in Grenoble, France. The sample was contained in a
4 2
Unit cell parameters for Mg(AlH ) and reliability factors
rotating 0.5 mm boron–silica–glass capillary. Data was col-
for the refinements at 8 K (PND), 111 and 295 K (combined
PND and PXD) are given in Table 1. Interestingly, the crys-
tallographic a-axis contracts somewhat upon heating from 8
to 295 K, ꢀa/a = −0.26%, whereas the c-axis shows a small
expansion, ꢀc/c = 0.25%. Table 2 lists the calculated atomic
coordinates and displacement parameters obtained from the
Rietveld refinements. The displacement factors for hydrogen
are high, even at 8 K, possibly due to the low decomposi-
◦
◦
lected between 2θ = 4.0 and 34.0 in steps of ∆(2θ) = 0.006 .
˚
The wavelength was 0.49956 A, obtained from a channel-cut
Si (111) monochromator. A temperature of 111 K was ob-
tained with a Oxford Cryostream series 600 cold nitrogen
blower.
Powder neutron diffraction (PND) data at 8, 111 and 295 K
◦
were collected from 10 to 130 in 2θ with the PUS instrument
at the JEEP II reactor at Kjeller (Norway) [18]. Monochro-
4 2
tion temperature of Mg(AlH ) . High thermal displacement
factors have also been observed for D in NaAlD [20] and
4
˚
˚
matized neutrons with λ = 1.5546 A (1.5554 A for the 8 K
measurements) were obtained from a Ge (511) focussing
monochromator. The detector unit consists of two banks of
LiAlD [21]. The fits from the combined PND and PXD Ri-
4
etveld refinements at 295 K are shown in Fig. 1. Note espe-
cially in Fig. 1a that although the hydride, rather than the
deuteride, was used in this study, the signal-to-noise ratio is
3
◦
seven position-sensitive He detectors, each covering 20 in
2
◦
θ (binned in steps of 0.05 ). The sample was placed in a
cylindrical V sample holder with 8 mm diameter. Data at
95 K were collected during sample rotation. Temperatures
of 8 and 111 K were obtained by means of a Displex cooling
4 2
surprisingly good. The space group of Mg(AlH ) is con-
¯
2
firmed to be P3m1.
The crystal structure of Mg(AlH ) is illustrated in Fig. 2.
4
2
◦
◦
system. The regions 76.30–77.55 and 115.60–117.70 were
excluded in the analysis of the 8 and 111 K PND data due
to additional scattering from the cooling system. Due to in-
coherent scattering from hydrogen, the PND measurements
were performed in high-intensity mode and with long mea-
surement times (2 days at 295 K and 4 days at 8 and 111 K)
to maximize the signal-to-noise ratio. The long measure-
ment times resulted in discontinuities in the diffractogram
at the changeover point between the different detector banks.
The structure can be viewed as consisting of isolated, slightly
−
distorted AlH4 tetrahedra. Each Mg atom is six-coordinated
Table 1
Refined unit cell parameters and reliability factors for Mg(AlH4)2, space
¯
group P3m1, Z = 4, at 8, 111 and 295 K
8
K
111 K
295 K
a ( A˚ )
c ( A˚ )
5.2084(3)
5.8392(5)
5.20309(12)
5.8400(2)
5.1949(2)
5.8537(2)
◦
◦
For that reason, the regions 69.50–70.50 , 89.80–90.40 and
1
◦
09.60–110.30 were excluded from further analysis for the
PND data.
Rietveld refinements were carried out with the program
Rwp (%)
PXD
PND
4.79
0.93
5.70
1.09
0.85
Fullprof (Version 2.50) [19]. The neutron scattering lengths
bMg = 5.38 pm, bAl = 3.45 pm and bH = −3.74 pm and X-ray
form factor coefficients were taken from the Fullprof library.
The PND and PXD data were given the weights 0.75 and
Rp (%)
PXD
PND
3.63
0.71
4.37
0.86
0.67
1.11
2
χ
1.72
1.53
Estimated standard deviations are given in parentheses.
0
.25, respectively, in the refinements. Pseudo-Voigt profile
Fossdal
