Chemistry Reference
In-Depth Information
Table 1 Structure types of the known clathrates
Group Clathrate
type
Code
IZA
Aristo
typ
e
General formula
Pearson
symbol
Hydrates Oxides Zintl
phases
|A
8
|[E
46
[4]
]
A
I
MEP
Pm
3
n
cP
54
þ
þ
þ
|A
24
|[E
136
[4]
]
II
MTN
Fd
3
m
cF
160
þ
þ
þ
DOH
P
6
/
mmm
|A
6
|[E
34
[4]
]
þ
þ
B
H
|A
15
|[E
120
[4]
]
DDR
R
3
m
þ
|A
4
|[E
88
[4]
]
NON
Fmmm
þ
|A
4
|[E
64
[4]
]
SGT
I
4
1
/
amd
þ
|A
2
|[E
12
[4]
]
SOD
Im
3
m
þ
þ
|A
20
|[E
40
[4]
]
AST
Fm
3
m
þ
P
4
2
/
mnm
|A
33
|[E
129
[4]
E
33
[3]
]
C
III
tP
195
þ
þ
|A
8
|[E
46
[4]
]
VIII
I
43
m
cI
54
þ
|A
24
|[E
68
[4]
E
32
[3]
]
þ
The three columns at the right-hand side indicate, by means of
cross marks
, whether these
clathrates exist as clathrate hydrates, oxides, or as Zintl phases
IX
P
4
1
32
cP
124
(aluminium phosphates) and related compounds (oxides) are given in the para-
graphs for the particular clathrate types. In these general formulas, the number of
oxygen atoms is not indicated. Only the relations between guest atoms or molecules
(A and M) and the usually tetra-connected atoms that correspond to the host
framework (E and T), respectively, are shown. The term A corresponds to guest
atoms in Zintl phases, whereas M corresponds to guest atoms in clathrasils. E and T
denote the atoms of the host framework in Zintl phases and tectosilicates, respectively.
As in other aluminates and silicates, several structural coincidences between the
cation subarrays in oxides and corresponding Zintl phases have been observed. This
is the case for the Group A clathrates, where the structure of the tetra-connected
atoms in silicides and germanides (Zintl phases) is maintained in the related
clathrasils and clathrate hydrates. The structural similarities between the type I
and II intermetallic clathrates, on the one hand, and the oxides melanophlogite and
dodecasil 3C, respectively, on the other, were already pointed out by O'Keeffe and
Hyde [
18
], whereas Wells, in his book “Structural inorganic chemistry”, showed
Despite the fact that the Group B clathrates, with the exception of the SOD clathrate
hydrate and the DOH clathrasils, do not show any coincidences, the group of
structures formed by the tetrahedral atoms of clathrasils of Group B may be
found as structures for Zintl-clathrates, and the structures of Zintl-clathrates of
group C may be metastable phases of the A
x
T
y
framework of clathrate oxides.
The structural analogy between clathrate hydrates and clathrasils (oxides)
deserves further discussion. Like all the crystalline phases of ice, the water mole-
cules of clathrate hydrates are hydrogen bonded to four neighbouring water mole-
cules in such a way that a H
2
O molecule has two additional hydrogen atoms linked
with each oxygen by hydrogen bonds. These hydrogen bonds are generated by the
difference in electronegativity between H and O atoms, which make water mole-
cules polar. The four hydrogen atoms around each oxygen atom have a pseudo-
tetrahedral configuration and their H-O-H bonds are arranged in a similar way as
the O-T-O bonds in the oxides of elements of Group 14 of the Periodic System of
