Chemistry Reference
In-Depth Information
spectroscopy and thermogravimetric analysis confirmed that the prepared compound con-
sists indeed of a hydrated aluminium fluoride, but the attempts to place water molecule in
the conventional cubic cell led to a poor agreement factor (R
Bragg
18 %). Water molecules
were finally considered as part of the first coordination sphere of Al
3
þ
ions. Nevertheless,
because water molecule is neutral, [AlF
6
x
(H
2
O)
x
]
x
3
octahedra display a lack of electron
density, which leads to consider the occurrence of cationic vacancies. The latter has been
clearly demonstrated by the convergence of the reliability factor down to R
Bragg
¼
4.3 %
when refining the occupancy rate of the Al
3
þ
atoms and introducing water in the anionic
fluorinated site (3a). The chemical formula Al
0.82
&
0.18
F
2.46
(H
2
O)
0.54
could be proposed
based on elemental analysis, TGA and Rietveld refinement. The crystallographic data of
Al
0.82
&
0.18
F
2.46
(H
2
O)
0.54
are gathered in Table 2.9 and compared to those of
-AlF
3
.
Interestingly, the interatomic distances for both hydrate and
-AlF
3
are very close. While
the occurrence of the water molecule should lead to an increase of the interatomic
distances as found for other aluminium fluoride hydrate,
61
cationic vacancies may act
as a repulsive entity decreasing the Al-X distances and maintaining the cubic symmetry.
Table 2.9 Crystallographic data of Al
0.82
&
0.18
F
2.46
(H
2
O)
0.54
,
a
-AlF
3
and the high temperature
cubic form
Phase
Crystal symmetry
Space Group Z
Unit cell para-
meters (
˚
)
Interatomic
distances(
˚
)Al-X
Angles (
)
Al
0.82
&
0.18
F
2.46
(H
2
O)
0.54
Cubic Pm-3m 1
3.6067(1)
1.8034(1)
180
-AlF
3
Rhombohedral R-3 c
12
4.9305(6)
12.4462(7)
1.797(3)
157.07(7)
High temperature cubic
form
Cubic Pm-3m 1
3.58
1.791
180
T=180
°
C
41
43
45
20
30
40
50
60
70
80
Figure 2.12 X-ray diffraction pattern of the aluminium fluoride hydrate obtained after
microwave irradiation at T ¼ 140
C. Inset: effect of the temperature on the XRD
