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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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Topotactic reactions

Topotactic reactions

Chapter:
(p.216) (p.217) 16 Topotactic reactions
Source:
Symmetry Relationships between Crystal Structures
Author(s):

Ulrich Müller

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199669950.003.0016

A topotactic reaction is a chemical solid-state reaction such that the orientations of the product crystals are determined by the orientation of the initial crystal. The same symmetry rules apply for the domains as for phase transitions. Crystals of C70 (P63/mcm) polymerize in the solid state; the space group of poly-C70 (Cmcm) is a subgroup of P63/mcm of index 3 and thus twins with three orientations are the result. Vernier compounds are halides of the lanthanoids with the general formula LnnX2n+1, e.g., Tm7Cl15. Their structures are hettotypes of the CaF2 type. When they crystallize from a melt having some intermediate composition, topotactic textures result consisting of, say, TmCl2 and Tm7Cl15. Their domains are intergrown such that the X-ray diffraction pattern is puzzling. However, with group-subgroup relations the puzzle can be solved.

Keywords:   topotactic reaction, solid-state reaction, vernier compounds

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