Chemistry Reference
In-Depth Information
Table 2.4 Structural characteristics of
a
-, HTB- and pyrochlore forms of AlF
3
Phase
Building
units
Al-X-Al (X¼ F or OH)
angles (
)
Average interatomic
distances (
˚
)
-AlF
3
4
158
1.797
-AlF
3
3 - 6 - 4
148- 148 - 166
1.800
Pyrochlore
3 - 6
141
1.842
Table 2.5 Crystallographic data obtained by Rietveld refinement
-AlF
2.6
(OH)
0.4
(HTB)
Pyrochlore AlF
1.7
(OH)
1.3
Crystal symmetry
Orthorhombic
Cubic
Space Group Z
Cmcm
Fd-3m
12
16
Unit cell
6.9681(2)
parameters (
˚
)
12.0360(3)
9.7309(1)
7.1434(1)
Volume (
˚
3
)
599
921
Interatomic distances(
˚
)
Al2-F1 21.857(12)
1.824(1)
Al1/Al2 - F2 41.77(3)/21.82(2)
Al1-F3 21.789(5)
Al2-F4 21.804(8)
Angles (
)
Al2-F1-Al2 139.4(5)
141.12(6)
Al1-F2-Al1 151.9(11)
Al1-F3-Al1 173.1(2)
Al2-F4-Al2 163.8(4)
Through the effect of the [HF]/[Al] molar ratio, it has been shown that the competition
between hydroxyl and fluoride groups has a strong influence in the stabilization of the final
compound. To understand the impact of the nature of the Al-X bond (X
¼
OH, F) on the
structure, conventional literature data (Table 2.4) are compared to those obtained by
Rietveld refinement for the compounds prepared by microwave irradiation (Table 2.5).
The impact of OH groups. Due to the strong electronegativity of fluoride ions, Al-F bond
displays shorter interatomic distance than Al-O(-OH) ones. This is well evidenced by the
bond distances of the different AlF
3
forms: 1.797 for
-AlF
3
, 1.80 for
-AlF
3
whereas the
hydroxyfluoride pyrochlore type structure, AlF
1.5
(OH)
1.5
, exhibits Al-X distances of
1.842
˚
. A slight variation of the chemical composition is sufficient to decrease the
interatomic distances because the pyrochlore with a chemical composition AlF
1.7
(OH)
1.3
exhibits distances of 1.824
˚
. The occurrence of OH groups has been also suggest to
favour small Al-X-Al angles as typically found in the three and six- membered rings.
55
The
impact of the OH groups inside the framework is relevant in the case of the HTB form.
Using conventional route, the HTB form contains some traces of OH groups while the
microwave synthesis leads to the preparation of an hydroxyfluoride phase. The compar-
ison between both compounds clearly highlights the impact of the OH groups upon the
structure. By comparison with the hydroxyl-free compound
-AlF
3
, it has been shown that
