Chemistry Reference
In-Depth Information
the two different building units, pentagonal dodecahedra and tetrakaidecahedra, in
dark and light grey shading, respectively. In this complex, three-dimensional
skeleton, all the E atoms of the Zintl phases and T atoms in the case of the oxides
have fourfold connectivity.
[5
12
]
[5
12
6
2
].
Cages per unit cell: 2
þ
6
Rings per unit cell: (48
5R)
þ
(6
6R).
Examples of this type of clathrate are:
|M
8
x
|[O
46
[4]
H
92
[2]
]
(a) Hydrates:
Clathrate-hydrates I
[
29
]
M
¼
CH
4
,N
2
,H
2
S, Ar
Xe,
...
...
|M
8
x
|[Si
46
[4]
O
92
[2]
]
(b) Oxides:
Melanophlogite
[
30
]
M
¼
CH
4
,N
2
,CO
2
,CH
3
NH
2
, Kr,
...
(c) Zintl phases:
|(K,Rb,Cs)
8
|[(Al,Ga,In)
8
(Si, Ge, Sn)
38
]
[
31
]
|(Na,K)
8
|[Si
46
]
[
32
]
|Eu
8
|[Ga
16
Ge
30
]
[
33
]
|(Sr, Ba)
8
|[(Al, Ga)
16
(Si, Ge, Sn)
30
]
[
34
]
|Ba
8
|[Cu
4
Si
42
]
[
35
]
|Ba
8
|[(Zn, Cd)
8
Ge
38
]
[
36
]
|I
8
|[Sn
10
In
14
P
22
], |I
8
|[Sn
14
In
10
P
21.2
□
0.8
]
[
37
]
|I
8
Br
8
|[Sn
17
Zn
7
P
22
], |I
8
Br
8
|[Sn
20
Zn
4
P
22
]
[
38
]
|I
8
|[Sb
8
Ge
38
]
[
39
]
|Br
8
|[Ge
38
As
8
]
[
40
]
|Te
8
|[Si
38
Te
8
]
[
41
]
|K
8
|[Ge
44
□
2
]
[
42
]
|(Rb, Cs)
8
|[Sn
44
□
2
]
[
43
]
In the Zintl phases |(K, Rb, Cs)
8
|[(Al, Ga, In)
8
(Si, Ge, Sn)
38
], |(Sr, Ba)
8
|[(Al,
Ga)
16
(Si, Ge, Sn)
30
], |Eu
8
|[Ga
16
Ge
30
], |Ba
8
|[Cu
4
Ga
4
Si
38
] and |Ba
8
|[(Zn, Cd)
8
Ge
38
],
the electropositive atoms - alkaline, alkaline-earth and rare-earth atoms - transfer
their valence electrons to the more electronegative atoms belonging to Groups 11,
12 and 13 of the Periodic Table (Cu, Zn, Cd, Al, Ga and In), converting them into
C
-Sn atoms. In this way, the Zintl-Klemm (ZK) concept explains
the fourfold connectivity of these atoms that, together with the Group 14 atoms,
form the three-dimensional host framework of the clathrates. In fact, the charge
must be considered to be transferred from the donor cations to the skeleton as a
whole, not specifically to an individual atom, giving rise in any case to a fourfold
connected skeleton. The electropositive atoms are placed in centres of both, the
pentagonal dodecahedra and the tetrakaidecahedra, constituting a subarray that is
similar to the structure of the intermetallic phases A15 (Cr
3
Si-type). This amazing
correspondence between structures of completely different atoms as Cr and Si in Cr
3
Si
The compounds |I
8
|[Ge
38
Sb
8
] and |Br
8
|[Ge
38
As
8
] can be called “inverse Zintl
electrons, but very electronegative atoms (Br and I atoms) that receive electrons
from the E framework. Thus, the eight halogen atoms would accept one electron each
(a total of eight e
) to complete their electron valence shell and convert the As and
Sb atoms into
-Si,
C
-Ge or
C
C
-Ge and
C
-Sn atoms, respectively. In this way, the tetra-connected
