Chemistry Reference
In-Depth Information
Elements. Thus, the structures of phases I
h
,I
c
and III of ice are similar to those of
three silica polymorphs: tridymite, cristobalite and keatite, respectively. Therefore,
it is not a surprise to find structural coincidences among clathrate hydrates and silica
clathrates, as we will see in the following section.
Next, the structures of different clathrates will be described, giving some
examples of each structural type. The descriptions are given in terms of the net of
tetra-connected atoms (E) which, in oxides, is the so-called “cation subarray” (T).
In addition to this, an analysis of the different cavities or cages and the number of
rings per unit cell is given. The cages are defined by the number of rings (faces) of
different size forming them, such that [5
12
] is the symbol for a cavity formed by 12
pentagons (pentagonal dodecahedron) and [5
12
6
2
] stands for a cavity having 12
pentagons and two hexagons.
3.1 Group A Clathrates
3.1.1 Type I Clathrates
The general composition of these clathrates is |M
2
M
0
6
|[(T
24
T
0
16
T
00
6
)
P
46
[4]
O
92
[2]
]
for oxides and |A
2
A
0
6
|[(E
24
E
0
16
E
00
6
)
P
46
[4]
] for Zintl phases. Here, M and M
0
and A
and A
0
are guest species enclosed in the cages of the structural framework. This
framework is formed by a basic building block called pentagonal dodecahedron
E
20
, which is usually described as [5
12
]. Two of these dodecahedra occupy the
origin and the centre of the unit cell and create interstices that are filled with other
polyhedral cavities that consist of 12 pentagons and two hexagons (tetrakaidecahe-
Fig. 2 Packing of polyhedra
formed by the host Si
Pentagonal dodecahedra [5
12
]
and tetrakaidecahedra [5
12
6
2
]
are depicted in
dark
and
light
grey
, respectively
