Chemistry Reference
In-Depth Information
Clathrate-like materials do not comprise an easily definable family of crystalline
solids and tectosilicates and tectoaluminosilicates, in particular, are very difficult to
classify. Whether or not a molecule or ion passes from one polyhedral void to the
next depends on the size of the windows formed by the oxygen atoms of the shared
polyhedra is less than six, only small ions such as Li
þ
can pass through. If the size
of the common face increases, larger species can move from one cavity to the
adjacent one. This could give rise to the formation of open channels, like those in
zeolite structures.
Therefore, a criterion is needed to distinguish porous from denser tectosilicates.
Usually, tetrahedral frameworks are classified according to their framework density
values of all the 4-connected tectosilicates whose structures are well established, a
gap is clearly recognised above 21 T-atoms per 1,000
˚
3
. Dense frameworks with
FD
21 have rather small voids that are either empty, as in cristobalite, or contain
barely or non-exchangeable cations, as in feldspars. Silicates with FD
>
21, have
<
larger cavities and channels and are classified as microporous.
The particular structure of these compounds leads to interesting physical proper-
ties such as cation exchange or molecule confinement and makes them useful for
industrial applications. Hydrates, for example, are denser than their typical fluid
hydrocarbons so that the gas molecules they contain are effectively compressed,
which offers a potential way to energy storage and transportation. Zeolites have a
wide range of utilities based on their selective cation-exchange ability (i.e. molec-
ular sieves, raw materials, etc.) and catalytic properties. They are used in different
fields such as oil industry, construction industry, water treatment or storage of
nuclear waste.
3 Similarities Between Clathrasils, Intermetallic Clathrates
and Clathrate Hydrates
This section provides structural descriptions of both intermetallic clathrates and
clathrasils, as well as similarities between them. The three-dimensional structures
of these compounds, particularly silicates, are usually difficult to describe. Here-
after, to simplify their description, they will be presented as formed by secondary
building units (SBUs). Rings of different sizes that are repeated along the structure
are considered as the most common SBUs. The sides of these polygonal rings can
be represented by the straight lines between neighbouring Si atoms, and the O
atoms would be placed near to the midpoints of these Si-Si lines. The SBUs give
rise to polyhedral building units that are the common constituents of clathrasil
frameworks.
The known types of clathrate structures are collected in Table
1
. In the fifth
column of the table, only the general formulas of the intermetallic clathrates (Zintl
phases) are given. Corresponding general formulas for the clathrasils, AlPOs
