Pauling s fifth rule

Exceptions from the rule usually show up as quantum-chemical effects which lower the total energies by also lowering the structural symmetries (see Section 3.4). On the other hand, we should add that there is a whole universe of amorphous structures which are deliberately excluded from the discussion if we follow Pauling s fifth rule but, admittedly, these refer to thennod)mami-cally metastable states in practically all cases. Indeed, the study of solid-state materials that do not exhibit translational invariance albeit chemical, local order - ordinary window glass is an ubiquitous example - has not been particularly excessive when compared with "normal" (that is, crystalline) solid-state materials, simply because the characterization of such matter is much more difficult for amorphous materials, ordinary X-ray or neutron diffraction loses its enormous analytical power. Thus, our atomistic knowledge of amorphous materials is far from being satisfactory, and Pauling s fifth rule should probably be taken with a pinch of salt. 

About 20 compounds with different kinds of anionic tetrahedron complexes in the same structure are considered in a special subdivision (Table 3). These structures do not obey Pauling s fifth rule. Belov s concept, emphasizing the dominating role of the cations, can be used for the interpretation of these structures with different tetrahedron complexes (Table 4). 

Fifth, the octahedral layers conform strictly to Pauling s rules in having shortened shared edges by counterrotations of the triads making up the two faces of the octahedra which are parallel to the sheet, and in being squashed in the direction normal to the sheets (Vhtch and Radoslovich [1963], Radoslovich [1963]). 

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