2004/09/25 | Pauling规则[Pauling's Rules]
类别(℃〖化学〗) | 评论(0) | 阅读(310) | 发表于 16:06
1. A coordination polyhedron of anions is formed about each cation, with the cation-anion distance being determined by the radius sum and the coordination number of the cation, and the cation by the radius ratio.
2. The electrostatic valency principle: In a stable crystal structure, the total strength of the valency bonds that reach an anion from all the neighboring cations is equal to the charge of the anion. The valence charge divided by the coordination number, called the electrostatic valency, is a measure of the strength of the bonds. Depending on the electrostatic valency, a crystal can be categorized as an isodesmic or anisodemic crystal.
3. The existence of edges, and particularly of faces, common to two anion polyhedra in a coordinated structure decreases its stability. This effect is large for cations with high valency and small coordination number, and is especially large when the radius ratio approaches the lower limit of stability of the polyhedron.
4.In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedral elements with each other.
5.The principle of parsimony. The number of essentially different kinds of constituents in a crystal tends to be small, because characteristically, there are only a few types of contrasting cation and anion sites. Thus, in structures with complex compositions, a number of different ions may occupy the same structural position (site). These ions must be considered as a single "constituent."
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