Chemistry Reference
In-Depth Information
Fig. 11 Host framework of
AST-type clathrates. In this
structure, the
lightly shaded
[4
6
6
12
]
cages
share the
hexagonal faces giving rise to
the formation of additional
cubic cages (
darkly shaded
ones
). The polyhedra
distribution is similar to the
structure of NaCl
These two types of cavities are arranged in the same way as the cations and anions
in the NaCl-type structure.
[4
6
]
[4
6
6
12
].
Cages per unit cell: 4
þ
4
Rings per unit cell: (24
4R)
þ
(24
6R).
Examples of this type of clathrate are:
(a) Oxides:
|(C
7
H
13
N)
4
(H
2
O)
16
|[Al
20
[4]
P
20
[4]
O
80
[2]
]
AlPO-16
[
61
]
|(C
7
H
12
N)
2
F|[Si
40
[4]
O
80
[2]
]
Octadecasil
[
62
]
Clathrate hydrates and Zintl phases of this type are unknown.
3.3 Group C Clathrates
These clathrates have only been found as hydrates and as Zintl phases. No oxide
with a similar T framework is known.
3.3.1 Type III Clathrates
This clathrate type, with ideal formula |A
20
A
0
10
|[E
172
[4]
], has a tetragonal unit cell
with 172 framework sites, which form pentagonal dodecahedra [5
12
], tetrakaideca-
[5
12
]
[5
12
6
2
]
[5
12
6
3
].
Cages per unit cell: 10
þ
16
þ
4
Rings per unit cell: (180
5R)
þ
(22
6R).
