Twin index
From Online Dictionary of Crystallography
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Indice de macle (Fr). Zwillingsindex (Ge). Indice di geminazione (It). 双晶指数 (Ja). Índice de macla (Sp).
Contents
Definition
A twin operation overlaps both the direct and reciprocal lattices of the individuals that form a twin; consequently, the nodes of the individual lattices are overlapped (restored) to some extent (twinning). The reciprocal n of the fraction 1/n of (quasi)-restored nodes is called the twin index.
Let (hkl) be the twin plane and [uvw] the lattice direction (quasi)-normal to it. Alternatively, let [uvw] be the twin axis and (hkl) the lattice plane (quasi)-normal to it. For twofold operations (180º rotations or reflections) the twin index is:
n = X/f, X = |uh+vk+wl|
where f depends on the lattice type and on the parities of X, h, k, l, u, v and w, as in the following table.
Lattice type | Condition on hkl | Condition on uvw | Condition on X | n |
---|---|---|---|---|
P | none | none | X odd | n = X |
X even | n = X/2 | |||
C | h+k odd | none | none | n = X |
h+k even | u+v and w not both even | X odd | n = X | |
X even | n = X/2 | |||
u+v and w both even | X/2 odd | n = X/2 | ||
X/2 even | n = X/4 | |||
B | h+l odd | none | none | n = X |
h+l even | u+w and v not both even | X odd | n = X | |
X even | n = X/2 | |||
u+w and v both even | X/2 odd | n = X/2 | ||
X/2 even | n = X/4 | |||
A | k+l odd | none | none | n = X |
k+l even | v+w and u not both even | X odd | n = X | |
X even | n = X/2 | |||
v+w and u both even | X/2 odd | n = X/2 | ||
X/2 even | n = X/4 | |||
I | h+k+l odd | none | none | n = X |
h+k+l even | u, v and w not all odd | X odd | n = X | |
X even | n = X/2 | |||
u, v and w all odd | X/2 odd | n = X/2 | ||
X/2 even | n = X/4 | |||
F | none | u+v+w odd | none | n = X |
h, k, l not all odd | u+v+w even | X odd | n = X | |
X even | n = X/2 | |||
h, k, l all odd | u+v+w even | X/2 odd | n = X/2 | |
X/2 even | n = X/4 |
When the twin operation is a rotation of higher degree about [uvw], in general the rotational symmetry of the two-dimensional mesh in the (hkl) plane no longer coincides with that of the twin operation. The degree of restoration of lattice nodes must now take into account the two-dimensional coincidence index Ξ for a plane of the family (hkl), which defines a super mesh in the twin lattice. Moreover, such a super mesh may exist in ξ planes out of N, depending on where is located the intersection of the [uvw] twin axis with the plane. The twin index n is finally given by
n = NΞ/ξ.
References
- Chapter 3.1.9 of International Tables for X-ray Crystallography (1959)
History
- Friedel, G. (1904). Étude sur les groupements cristallins. Extrait du Bullettin de la Société de l'Industrie minérale, Quatrième série, Tomes III et IV. Saint-Étienne, Société de l'imprimerie Thèolier J. Thomas et C., 485 pp.
- Friedel, G. (1926). Leçons de Cristallographie. Berger-Levrault, Nancy, Paris, Strasbourg, XIX+602 pp.
See also
- Chapter 1.3 of International Tables for Crystallography, Volume C
- Chapter 3.3 of International Tables for Crystallography, Volume D