RESPONSE TO CHANGES IN T, P OR CHEMISTRY: FRAMEWORK MECHANICS OF ALUMINOSILICATES. Werner H. Baur, Institut für Kristallographie, Senckenberganlage 30, D-60054 Frankfurt am Main, Germany
Since in situ experiments under extreme conditions have become more common it is possible to follow up the detailed changes going on in the frameworks of porous materials. Such frameworks adapt to physical (temperature and/or pressure) and chemical (cation exchange) influences by changing the geometry of the T-O-T angles and T-O bonds and by undergoing phase transitions. Nevertheless, despite the adjustments in geometry some of the frameworks remain stable und even preserve approximately their overall dimensions. Frameworks formed by TO4 coordination tetrahedra are composed of rigid parts, the tetrahedra, and of bridging O atoms serving as flexible hinges between the tetrahedra. A few tetrahedral frameworks are noncollapsible: their hinges antirotate, that is compression at one hinge necessitates tension at another hinge and vice versa (1). This happens to be true for the LTA and FAU types and for the feldspar framework. These frameworks do not collapse, even if no cations or sorbed molecules are present in their pores. On the other hand in collapsible frameworks the hinges corotate and compression or tension at all hinges is exerted simultaneously in the same sense. Collapse in such cases is stopped either (a) by the ions and molecules within the pores, when the framework begins to hug its filling, or else (b) when the collapse cannot proceed because the angles at the hinges T-O-T assume values which are too small to be compatible with the necessary bonding requirements [as is observed at 598 K in the dehydrated form of natrolite, metanatrolite, where Si-O-A1 angles of 114deg. occur (2)]. The responses of the frameworks to change will be illustrated by examples, particularly stressing the mechanics of the feldspar type (3).
(1) Baur, W. H. (1992). J. Sol. State Chem. 97, 243-247
(2) Baur, W. H. & Joswig, W. (1996) N. Jahrb. Min. Mh., in the press
(3) Baur, W. H. et al. (1996) J. Solid State Chem., in the press