Structural Relationships Between Intermetallic Clathrates, Porous Tectosilicates and Clathrate Hydrates - Inorganic 3D Structures - page 2

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Nomenclature and Importance of Clathrate-Type Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Similarities Between Clathrasils, Intermetallic Clathrates and Clathrate Hydrates . . . . . . . 6
3.1 Group A Clathrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Group B Clathrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3 Group C Clathrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 Similarities Between Zeolites and Intermetallic Clathrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1 ABW-Type Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2 BCT-Type Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5 Considerations on the Stability of Intermetallic Clathrates and Clathrasils . . . . . . . . . . . . . . 24
6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Abbreviations

EZKC Extended Zintl-Klemm concept
FD Framework density

s Connectedness

SBU Secondary building unit

vec Valence electron concentration

1 Introduction

Tectoaluminosilicates contain three-dimensional frameworks built from [TO 4 ]
tetrahedra, where T is Al or Si. Some of them are structurally rather complex.
Usually, these compounds contain networks of corner-sharing [TO 4 ] tetrahedra,
where all the corners are shared between two tetrahedra (connectedness s 4),

although some cases with lower connectedness are also known (some tetrahedra in
the structure share less than four corners). To this day, richly illustrated mono-
graphs have described and classified the structures of crystalline tectosilicates
[ 1 - 3 ]. The structures of aluminates and silicates have often been described as
more or less distorted dense packings of oxygen anions O 2 in which Al 3 þ and
Si 4 þ cations occupy part of the interstitial tetrahedral voids, and more electroposi-
tive cations, such as alkaline, alkaline-earth or rare-earth, are placed in larger voids
[ 1 , 2 ] . Thus, this descriptive model considers that tectosilicates are formed by the
condensation of [SiO 4 ] tetrahedra, but no explanation on the rich variety of poly-
anions is given. Although this type of compounds has been widely studied, the
reasons why they adopt a particular structure are still not well known.

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