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Difference between revisions of "Twinning (endemic conditions of)"

From Online Dictionary of Crystallography

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'''Cubic lattices'''
 
'''Cubic lattices'''
  
The primitive cells of ''cF'' and ''cI'' lattices are rhombohedric (''hR'') lattices with α = 60° and 109.47°, respectively; a ''cP' lattice can be seen an a rhombohedric lattice with α = 90°. As said above, a ''hP'' sublattice is always embedded in a ''hR'' lattice with consequent possibility of favouring twinning by reticular merohedry.  
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The primitive cells of ''cF'' and ''cI'' lattices are rhombohedric (''hR'') lattices with α = 60° and 109.47°, respectively; a ''cP'' lattice can be seen an a rhombohedric lattice with α = 90°. As said above, a ''hP'' sublattice is always embedded in a ''hR'' lattice with consequent possibility of favouring twinning by reticular merohedry.  
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'''Pseudo cubic lattices'''
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The primitive cells of ''oI'' and ''tI'' lattices always show ''a'' = ''b'' = ''c''
  
 
[[Category:Twinning]]
 
[[Category:Twinning]]

Revision as of 12:14, 16 May 2006

Geminazione (condizioni endemiche di) (It)

When a lattice necessarily contains at least one sublattice that supports either twinning by reticular merohedry or twinning by reticular pseudomerohedry, it is said that an endemic condition of twinning does exist. The following cases are known.

Rhombohedral lattice (hR)

A hR lattice (symmetry [math] \bar 3[/math]m) always contains a hP lattice (symmetry 6/mmm). Consequently, the crystal structures based on a hR lattice are endemic candidates to twinning by reticular merohedry via the symmetry elements that occur in the 6/mmm point group of the sublattice, but not in the [math] \bar 3[/math]m point group of the lattice.


Cubic lattices

The primitive cells of cF and cI lattices are rhombohedric (hR) lattices with α = 60° and 109.47°, respectively; a cP lattice can be seen an a rhombohedric lattice with α = 90°. As said above, a hP sublattice is always embedded in a hR lattice with consequent possibility of favouring twinning by reticular merohedry.

Pseudo cubic lattices

The primitive cells of oI and tI lattices always show a = b = c