Chemistry Reference
In-Depth Information
Fig. 12 Polyhedral host
framework of the Zintl phase
The different shading
indicates the different types
of polyhedra present in the
structure (
medium grey
for
[5
12
] pentagonal
dodecahedra,
dark grey
for
[5
12
6
3
] pentakaidecahedra
and
unshaded
for [5
12
6
2
]
tetrakaidecahedra). Six
percent of the Sn positions are
vacant in the structure, as
explained in the text
Examples of this type of clathrate are:
(a) Hydrates:
|((C
4
H
9
)
4
N
C
6
H
5
COO)
4
|[O
172
[4]
H
344
[2]
]
Clathrate-hydrate III
[
63
]
(b) Oxides:
Unknown
|Cs
30
Na
2.8
|[Sn
129.6
[4]
Sn
32.8
[3]
(c) Zintl phase:
□
9.6
]
[
64
]
In this Zintl phase, the electrons transferred from the Cs and Na atoms produce a
framework with a valence electron concentration higher than 4 e
/atom. This
excess of electrons in the E framework gives rise to an underlinked structure.
Thus, the Cs and Na atoms transfer almost 33 electrons to an equal number of Sn
atoms, transforming them into
-Sb atoms are tri-
connected and 9.6 vacancies are generated in the idealised fourfold connected
framework, Cs
30
Na
2.8
|[Sn
162.4
□
9.6
]. Note that, in this compound, the number of
electropositive atoms slightly exceeds the number of available cavities of the ideal
|A
20
A
0
10
|[E
172
] compound.
C
-Sb atoms. As a result, these
C
3.3.2 Type VIII Clathrates
This clathrate type is known only as Zintl phase with the general formula |A
8
|
[(E
24
E
0
12
E
00
8
E
000
2
)
P
46
[4]
]. It has a complex structure in which the more electroneg-
ative atoms (E) have a fourfold connectivity. This is easily deducible from the
Zintl-Klemm concept. The structure consists of a spatial assemblage of [5
6
6
6
]
building units: clusters E
8
and stuffed tetrahedra E@(E
4
).
Cages per unit cell: 8
[5
6
6
6
].
Rings per unit cell: (24
5R)
þ
(24
6R).
