superstructure (rus. сверхструктура) — structural disturbance of a crystalline compound or alloy repeated with certain regularity thus creating a new structure with another alternation period. Basic cell of such structure – supercell – is usually a multiple of an elementary cell of the original structure.

Description

The term "superstructure" was introduced to describe the structure of ordered solid solutions. Superstructures are formed below a certain temperature (called the "order-disorder transition temperature") in cases where atoms of a certain sort are energetically preferable to be surrounded by atoms of another sort. Superstructures often originate as a result of second-order phase transitions. The formation of superstructures is accompanied by the appearance of weak additional superstructure lines in diffraction patterns used to detect and identify the type of superstructures.

One example of a superstructure is the structure of Cu-Zn alloy (brass) where in the disordered state Cu and Zn atoms are distributed with equal probability over the nodes of the body-centred lattice, and in the ordered state atoms of one type occupy nodes at the corners of cubic cells and atoms of the other type – in their centres.

Often the term "superstructure" is used to denote a structure where the growth of lattice defects (impurity atoms, vacancies, etc.) makes these defects orderly placed. Normally, this ordering is accompanied by reduction in the crystal lattice symmetry (see the Figure).

Illustrations

Development of different types of superstructures with doubled elementary cell parameters by exam

Development of different types of superstructures with doubled elementary cell parameters by example of a volume-centered crystalline lattice. 

а) original structure elementary cell; 

b) replacement of half of cations in the original structure by other cations with ordered alternation of both cation types; 

c) replacement of half of anions in the original structure by other anions with ordered alternation of both anion types; 

d) replacement of half of anions in the original structure by anion vacancies with ordered alternation of anions and vacancies; 

e) replacement of half of cations in the original structure by  vacancies with ordered alternation of cations and vacancies; 

f) ordered displacement of half of cations in the original structure to the left and of half of cations to the right.


Author

  • Shlyakhtin Oleg A.

Sources

  1. Superstructure (in Russian)// Metallurgicheskijj slovar'. — www.metallobaza.com/abc/s/1696.htm (reference date: 12.12.2011).
  2. Solodovnikov S.F. Key terms and concepts of structural crystallography: Guidebook. (in Russian) — Novosibirsk: INH SO RAN, 2005. — 113 pp.

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