Lecture #8 Isomorphism and polymorphism Plan

LECTURE #8


Isomorphism and polymorphism
Plan:
1. The history of isomorphism and polymorphism .
2. Conditions necessary for the manifestation of isomorphism .
3. Classification of polymorphism types by structure .

The concept of isomorphism was introduced to science by E. Mitscherlich in 1819 , according to which KH ₂ PO ₄ and KH ₂ AsO ₄ it is noted that it is a complex form consisting of tetragonal prism and dighramide belonging to tetragonal syngonia to the same shape of crystals. The angle between the (101) and (011) sides of this crystal is 58°, and as a result of further measurements, it is determined that this angle is 57°58 for potassium phosphate and 57°52 for potassium arsenate. At the same time, the ratio of crystallographic axes turned out to be 1:0.939 and 1:0.938, which indicates that the crystals of these substances are very similar to each other, but one is not the same as the other.

E.Mitcherlich called substances with the same or almost the same shape of different crystals chemically, and physically similar substances called isomorphous substances.
Initially, it was considered that isomorphous substances differed from one another in terms of their chemical composition when the same atoms were replaced by other atoms in a ratio of 1:1. This theory helped to determine the atomic weights of a number of elements in its time, in the first half of the 19th century. In particular, the isomorphism of KMnO ₄ with KClO ₄ made it possible to determine the atomic weight of Mn (relative to chlorine). Later, after the development of methods for examining the internal structure of crystals using X-rays, the phenomenon of isomorphism and its nature were determined in some detail.
The property of forming a mixed crystal of substances with different chemical composition is called isomorphism. In this case, atoms and ions of a substance forming an isomorphous mixture can exchange places with atoms and ions of another substance in different proportions. As a result of this exchange, mixed crystals are formed, such crystals are sometimes called solid solutions.
Substances with different crystal structures can form isomorphous compounds in two different ways. They are called isovalent and heterovalent isomorphisms. In isovalent isomorphism, ions in the crystal structure can exchange places only with ions of equal valence. In this case, the difference between the radii of these alternating covalent ions must not exceed 15%, otherwise the stability of the crystal structure will be lost. For example, from divalent cations Mg ²⁺ (ionic radius 0.74 A°); Fe ²⁺ (0.80A°); Na ²⁺ (0.74 A°); Zn ²⁺ (0.83 A°) and Mn ²⁺ (0.91 A) trivalent cations Fe ³⁺ (0.64A°), Cr ³⁺ (0.64 A°); Among the anions Al ^{3+ (0.57 A°)} , elements such as − S ^2- (1.82 A°) and Se ^{2- (1.93 A°) are isomorphous with each other in chemical compounds with different crystals.} can exchange.
In isovalent isomorphism, the number of exchangeable system units in an isomorphic mixture remains unchanged. For example, Fe with Mg in the isomorphic series MgCO3 • FeCO _{3 ;} In the CuS - CuSe isomorph series, S and Se can exchange places with an equal number of ions.