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Symmetry Relationships between Crystal StructuresApplications of Crystallographic Group Theory in Crystal Chemistry$
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Ulrich Müller

Print publication date: 2013

Print ISBN-13: 9780199669950

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199669950.001.0001

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Crystal structures of molecular compounds

Crystal structures of molecular compounds

Chapter:
(p.185) 14 Crystal structures of molecular compounds
Source:
Symmetry Relationships between Crystal Structures
Author(s):

Ulrich Müller

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199669950.003.0014

Even though molecular packings often have low symmetries, they often can be regarded of derivatives of high-symmetry structures. Sometimes the molecules are arranged in a similar way as in a packing of spheres, but with reduced space-group symmetry so that the site symmetry of a molecule is compatible with its symmetry. The space group then is a corresponding subgroup of the space group of the sphere packing. Cage-like molecules like P4S3 adopt several subgroups of P63/mcm, all with packings like in a hexagonal closest packing of spheres. This sometimes even applies to molecules that are far from being spherical. Tetraphenylphosphonium salts crystallize in the space group P4/n if the anion has a fourfold rotation axis. If the symmetry of the anion is lower, the kind of packing often is kept, but the space group is a subgroup of P4/n.

Keywords:   molecular compounds, cage-like molecules, tetraphenylphosphonium salts

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