Chemistry Reference
In-Depth Information
Fig. 4 Cs subarray in the type-II clathrate compound |Cs
8
Na
16
|[Si
136
][
50
]
. In this structure, the
[5
12
6
4
] hexakaidecahedra share hexagonal faces forming a diamond-type skeleton
the [5
12
6
4
] polyhedra are the interstices of the compact packing. All the E and T
atoms are tetra-connected. An alternative description to the ordering of the [5
12
6
4
]
polyhedra in this structure was proposed by Higgins [
46
], who pointed out that these
polyhedra form a diamond-like lattice connected through six-ring faces (Fig.
4
).
[5
12
]
[5
12
6
4
].
Cages per unit cell: 16
þ
8
Rings per unit cell: (144
5R)
þ
(16
6R).
Examples of this type of clathrate are:
(a) Hydrates:
|M
16
x
M
0
8
x
|[O
136
[4]
H
272
[2]
]
Clathrate-hydrates II
[
47
]
|(N
2
)
16
x
(C
6
H
11
N)
8
y
|[Si
136
[4]
O
272
[2]
]
(b) Oxides:
Dodecasils 3C
[
48
]
(c) Zintl phases:
|Ba
16
8
|[Ga
32
Sn
104
]
[
49
]
□
|Cs
8
Na
16
|[(Si, Ge)
136
]
[
50
]
Clathrates of Type II are new examples of structural similarity between the
substructures of oxides (Dodecasil 3C) and the structures of related Zintl phases.
With regard to the Zintl phases presented in this section, they have the same
behaviour as those described in the previous one. In the compound |Ba
16
□
8
|
[Ga
32
Sn
104
], the tetraconnectivity of the E framework can be explained by means
of the ZK concept, since the valence electrons of the Ba atoms would convert the
Ga atoms into
-Ge. Nevertheless, in the compounds |Cs
8
Na
16
|[(Si, Ge)
136
] there
exists an excess of electrons. This fact can be related, as it was mentioned before,
either to a delocalisation of these electrons over the bonds of the skeleton or to the
existence of vacant E positions. An experimental evidence that supports the first
hypothesis is the metallic behaviour of these compounds, compared to the semi-
conducting behaviour of |Ba
16
□
8
|[Ga
32
Sn
104
]. The structure of the compound
C
