Chemistry Reference
In-Depth Information
eight-membered rings in the ratio 2:8:4. The framework presents octagonal chan-
nels along the [001] direction with a pore diameter of approximately 2.4
˚
.
[4
2
6
4
]
[6
2
8
2
].
Cages per unit cell: 2
þ
4
Rings per unit cell: (2
4R)
þ
(8
6R)
þ
(4
8R).
Examples of this type of zeolite are:
(a) Oxides:
Metavariscite
AlPO
4
·2H
2
O
[
72
]
|K
4.56
|[Mg
2.28
[4]
Si
5.72
[4]
O
16
[2]
]
BCT-type zeolite
[
73
]
(b) Binary compounds:
CrB
4
[
74
]
b
-BeO
[
75
]
No clathrate hydrate with BCT framework has been observed.
The framework of |K
4.56
|[Mg
2.28
[4]
Si
5.72
[4]
O
16
[2]
] is also consistent with the
EZKC. This means that a
-Si array is formed when potassium atoms donate
their valence electrons to the [Mg-Si] skeleton. In this way, its structure is similar to
the AlP array in metavariscite, a
C
C
-Si skeleton formed by an equal amount of atoms
of the Groups III and V.
The tetrahedral framework of these zeolite-like compounds is strongly related to
potential structures of the Group 14 elements. Thus, this array is similar to the boron
network in the intermetallic compound CrB
4
and to the structure of the binary oxide
b
-BeO, a II-VI compound. Although no element of Group 14 with this type of
structure is known, it is remarkable that a Zintl phase such as CrB
4
, where the boron
atoms could be converted into
C
-C, and a II-VI compound can adopt this same type
of structure.
5 Considerations on the Stability of Intermetallic Clathrates
and Clathrasils
An analysis of the SBUs (rings) that form the different polyhedra of the structures
of intermetallic clathrates and clathrasils can give us some insight into the stability
of these compounds. Most of the rings of these skeletons are quasi-planar. This fact
becomes evident by analysing the torsion angles of all the five- and six-membered
rings present in these structures. Moreover, the angles
E-E-E in the intermetallic
clathrates and
∠
T-T-T in the clathrasils are close to those of the different regular
polygons: 60
for the triangle, 90
for the square, 108
for the pentagon and 120
for
the hexagon. The exception is the NON-type clathrasil, whose rings deviate con-
Consider now the E frameworks in intermetallic clathrates. They mainly consist
of Group 14 or pseudo-Group 14 elements that are tetra-connected. Si atoms
in these clathrates, for instance, have a strong hybridisation of the 3
s
orbital with
the three 3
p
orbitals to form four equivalent lobes of a
sp
3
hybrid orbital, all
pointing towards the corners of a regular tetrahedron. In this case, the Si atoms
form directed bonds with angles close to the tetrahedral value of 109.5
(the ideal
∠
