Chemistry Reference
In-Depth Information
Fig. 13 Structure of the
clathrate VIII compound |Ba
8
|
by [5
6
6
6
] cages of Sn and Ga
atoms at the polyhedron
vertices and the Ba atoms in
the centres of the cavities
Examples of this type of clathrate are:
(a) Zintl phases:
|Ba
8
|[Ga
16
Sn
30
]
[
34
]
|Eu
8
|[Ga
16
Ge
30
]
[
33
]
Hydrates and oxides of type VIII are unknown.
3.3.3 Type IX Clathrates
Zintl phases of this clathrate type have a general formula |A
12
A
0
8
A
00
4
|
[(E
24
E
0
24
E
00
12
E
000
8
)
P
68
[4]
(E
24
E
0
8
)
P
32
[3]
] and a tetragonal structure characterised
by a three-dimensional framework of condensated E
20
pentagonal dodecahedra
(Fig.
14
). The electropositive A atoms are located in a narrow 3D channel labyrinth.
[5
12
] and big cavities formed by 5R, 11R and 12 R.
Cages per unit cell: 8
Rings per unit cell: (84
5R)
þ
(24
11R)
þ
(24
12R).
Examples of this type of clathrate are:
(a) Zintl phases:
|K
24
[Sn
92
Bi
8
]
[
65
]
|Ba
24
|[Ge
84
In
16
]
[
66
]
|K
32
|[Sn
100
]
[
67
]
|Ba
29
x
Eu
x
|[Ge
100
]
[
68
]
Clathrate hydrates and oxides of type IX are unknown.
All the Zintl phases of this type, except the last one, have polyanions with
pseudo-stoichiometry
17:8), the
formation of which is explained by the formal transference of electrons from the
more electropositive atoms. The last example, however, has an excess of electrons
C
-Si
68
P
32
(ratio of tetra-/tri-connected atoms
¼
