The Polling rules are two crystal chemical laws on the structure of crystals . First formulated by Linus Pauling .
Many crystals have a structure based on two closest packages: the densest cubic package and the densest hexagonal package . In this case, the densest ions are formed by large ions , and usually these are anions, and small ions are located in three types of voids: in octahedral , tetrahedral, and triangular positions.
Pauling's rules are simple and logical, but they summarize the patterns that are implemented on most structures, and bring great benefit in the study of crystals.
Pauling's first rule
Pauling 's first rule states that in the closest packs large (if the ratio of the cation to anion radius is greater than 0.414) cations are located in octahedral voids, smaller cations occupy tetrahedral positions (the ratio of radii from 0.414 to 0.215) and small (less than 0.215) triangular.
Pauling's second rule
Pauling's second rule requires that in a stable structure the local balance of valencies be observed, that is, the sum of the valence forces (the ratio of the valence of an atom to its coordination number ) of all the nearest neighbors of a given atom would be equal to or approximately equal to its own valence. This rule makes it unlikely to fill positions with a large deviation from the local balance (more than 10-20%).
See also
- Crystal cell