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Oriented association and twinning

Crystals (also called individuals or components) or domains belonging to the same phase form an oriented association if they can be brought to coincidence by a translation, rotation, inversion or reflection. Individuals related by a translation form a parallel association; domains related by a translation form antiphase domains. Individuals or domains related by a reflection, inversion or rotation form a twin called, respectively, reflection twin, inversion twin or rotation twin.

A mapping relating differently oriented crystals cannot be a symmetry operation of the individual: it is called a twin operation and geometric element about which it is performed, associated with this operation, is called twin element. Mallard's law states that the twin element is restricted to a direct lattice element: it can thus coincide with a lattice node (twin centre), a lattice row (twin axis) or a lattice plane (twin plane).

The symmetry of a twin (twin point group) is obtained by extending the intersection point group of the individuals in their respective orientations by the twin operation.

Classification of twins

Twins are classified following Friedel's reticular (i.e. lattice) theory of twinning (see: G. Friedel Lecons de Cristallographie, Nancy (1926) where reference to previous work of the author can be found; see also Friedel's law). This theory states that the presence, either in the lattice or a sublattice of a crystal, of (pseudo)symmetry elements is a necessary, even if not sufficient, condition for the formation of twins. In presence of the reticular necessary conditions, the formation of a twin finally still depends on the matching of the crystal structures at the contact surface between the individuals.

The following categories of twins are described under the listed entries.

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 = Oriented association and twinning =
-Crystals (also called individuals or components) or domains belonging to the same phase form an oriented association if they can be brought to the same crystallographic orientation by a translation, rotation or reflection. Individuals related by a translation form a ''parallel association''; strictly speaking these individuals have the same orientation even without applying a translation. Individuals related by a reflection [either plane (''[[reflection twin]]'') or centre (''[[inversion twin]]'') of symmetry] or a rotation (''[[rotation twin]]'') form a ''twin''.
+Crystals (also called ''individuals'' or ''components'') or domains belonging to the same phase form an oriented association if they can be brought to coincidence by a translation, rotation, inversion or reflection. Individuals related by a translation form a ''parallel association''; domains related by a translation form ''antiphase domains''. Individuals or domains related by a reflection, inversion or rotation form a twin called, respectively, [[reflection twin]], [[inversion twin]] or [[rotation twin]].
-'''symmetry of a twin''' - See ''[[Eigensymmetry]]''
+A [[mapping]] relating differently oriented crystals cannot be a symmetry operation of the individual: it is called a [[twin operation]] and [[geometric element]] about which it is performed, associated with this operation, is called [[twin element]]. [[Mallard's law]] states that the twin element is restricted to a [[direct lattice]] element: it can thus coincide with a lattice node (''twin centre''), a lattice row (''twin axis'') or a lattice plane (''twin plane'').
-An element of symmetry crystallographically relating differently oriented crystals cannot belong to the individual. The element of symmetry that relates the individuals of a twin is called ''[[twin element]]'' and the connected operation is a ''[[twin operation]]''. The ''[[Mallard's law]]'' states that the ''twin element'' (i.e. the geometrical element relative to which the twining operation is defined) is restricted to a [[direct lattice]] element: lattice nodes (''twin centres''), lattice rows (''twin axes'') and lattice planes (''twin planes'').
+The symmetry of a twin (''twin point group'') is obtained by extending the intersection [[point group]] of the individuals in their respective orientations by the twin operation.
-In most twins the symmetry of a twin (''twin point group'') is that of the individual point group augmented by the symmetry of the twinning operation; however, a symmetry element that is oblique to the [[twin element]] is absent in the twin (e.g., ''spinel twins'': ''m''<math> \bar 3</math>''m'' crystal point group; {111} [[twin law]]; <math> \bar 3</math>/''m'' twin point group).
 = Classification of twins =

Difference between revisions of "Twinning"

From Online Dictionary of Crystallography

Revision as of 15:38, 9 April 2015

Contents

Oriented association and twinning

Classification of twins

Related articles

See also