Isomorphous crystals

Cristaux isomorphes (Fr). Isomorphe Kristalle (Ge). Cristalli isomorfi(It). 同型の結晶 (Ja). Cristales isomorfos (Sp).

Definition

Two crystals are said to be isomorphous if (a) both have the same space group and unit-cell dimensions and (b) the types and the positions of atoms in both are the same except for a replacement of one or more atoms in one structure with different types of atoms in the other (diadochy), such as heavy atoms, or the presence of one or more additional atoms in one of them (isomorphous addition). Isomorphous crystals can form solid solutions.

History

The notion of isomorphism was discovered by Eilhard Mitscherlich (1794–1863) who found that the crystal forms of salts such as the hydrated potassium phosphates and arsenates or the hydrated potassium copper and iron sulfates were identical (Mitscherlich, 1819, 1820, 1821, 1823).

References

• Mitscherlich, E. (1819). Abh. K. Akad. Wiss. Berlin, pp. 427–437. Über die Kristallisation der Salze, in denen das Metall der Basis mit zwei Proportionen Sauerstoff verbunden ist
• Mitscherlich. E. (1820). Ann. Chimie Paris, 14, 172–190. Sur la relation qui existe entre la forme cristalline et les proportions chimiques. Part I
• Mitscherlich. E. (1821). Ann. Chimie Paris, 19, 350–419. Sur la relation qui existe entre la forme cristalline et les proportions chimiques. Part II
• Mitscherlich. E. (1823). Ann. Chimie Paris, 24, 264–271. Sur la relation qui existe entre la forme cristalline et les proportions chimiques. Part III

See also

• Isomorphous replacement
• Morphotropism
• Chapter 2.4 of International Tables for Crystallography, Volume B