Structural Relationships Between Intermetallic Clathrates, Porous Tectosilicates and Clathrate Hydrates - Inorganic 3D Structures - page 4

these structures in the light of both the extended Zintl-Klemm concept (EZKC)
[ 3 , 6 , 20 , 21 ] and Pearson's generalised octet rule [ 22 ] . In aluminosilicates of highly
electropositive metals, the Si and Al atoms form three-dimensional skeletons which
can be interpreted as if Si ( Al ) atoms were behaving as Zintl polyanions , adopting
the structure of either main-group elements or isoelectronic Zintl polyanions,
showing the same connectivity [ 3 , 6 ] . That is, the more electropositive cations
seem to transfer their valence electrons to the tetrahedral atoms, converting them
into pseudo-atoms, such as pseudo-Si (

C -Si), pseudo-phosphorus ( C -P) or pseudo-

sulphur ( -S). The oxygen atoms are then located close to both the hypothetical

two-electron bonds and the lone pairs of the cations, giving rise to the tetrahedral
coordination of silicon or aluminium. From this, it was concluded that the coordi-
nation sphere of Si and Al and the global three-dimensional structure of the
aluminosilicates depend on the nature of the other cations accompanying Si and
Al in the structure [ 3 , 6 ] .
In this report, we describe a comprehensive and comparative study of the
structures of both, the intermetallic clathrates and porous tectosilicates (clathrasils
and some zeolites), to show that their skeletons obey the same general principles.
The structures of porous tectosilicates will be discussed in terms of the EZKC and
Pearson's octet rule, showing that oxides adopt structures similar to those of the
corresponding intermetallic compounds. Hydrates, which adopt similar skeletons to
clathrasils, will be studied in detail also. Before discussing the clathrate structures,
it will be useful to review information concerning the nomenclature and the
importance of this family of compounds. These aspects will be covered in
Sect. 2 . Subsequently, the different structures will be described and discussed,
and the comparison of both, the clathrasil and zeolite compounds with the interme-
tallic-clathrate compounds will be presented in Sects. 3 and 4 , respectively. Finally,
we will present some considerations on the stability of this type of compounds.

C

2 Nomenclature and Importance of Clathrate-Type
Compounds

Clathrate-type compounds have attracted the attention of scientists since they were
discovered. At the beginning of the nineteenth century, Sir Humphry Davy first
showed that water was the main component of a solid substance that was previously
considered as solid chlorine [ 23 ] . Subsequent experiments showed that this new
phase has an approximate composition Cl 2

10H 2 O and is the first crystalline
clathrate hydrate known. From this moment, clathrate hydrates of many substances,
including rare gases and hydrocarbons, have been found [ 24 ].
Clathrate hydrates belong to a large group of compounds, which consist of three-
dimensional frameworks (host structures) trapping different molecules or atoms
(guest species). The term “clathrate” was coined in 1948 by Powell [ 25 ], who
discovered an inclusion compound, where guest atoms were enclosed in cavities