Four-Membered Rings, frames

The stmcture of cordierite-like materials is based on a frame of font- and six-member rings composed of aluminum-silicon-oxygen tetrahedral. Mg cations are localized in octahedrons between tetrahedrons of six- and four-member rings. A frame stmcture allows to obtain modified materials with 3d cations instead of Mg having similar ionic radiuses, for example, Mn, Co Ni, Cu, Zn [4]. The presence of transition elements is of interest because these elements may strongly influence on the cordierite properties and catalytic activity. 

A last important feature concerns the bicyclic species. Our calculations show that their formation is favoured when both constitutive rings contain each an even number of tetrahedra. The concentration of oligomers made of two four- or six-membered rings is respectively larger than the concentration of species containing two three- or five- membered rings. This behaviour is directly related to Loewenstein1 s rule and can be related to the fact that the frame-... 

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