On the Sequence of Phase Transitions in Tridymite

Abstract We consider the phase transitions in tridymite from the perspective of the rigid unit mode model. The rigid unit modes are the low-frequency phonons of a crystal structure that consists of an infinite framework of tetrahedra linked at corners, that can propagate without the tetrahedra disto...

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Bibliographic Details
Published in:Physics and chemistry of minerals 1998-12, Vol.26 (2), p.171-179
Main Authors: Pryde, A. K. A., Dove, M. T.
Format: Article
Language:English
Subjects:
Crystal structure
Distortion
Energy costs
High temperature
Low temperature
Phase transitions
Phases
Tetrahedra
Tridymite
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Summary:Abstract We consider the phase transitions in tridymite from the perspective of the rigid unit mode model. The rigid unit modes are the low-frequency phonons of a crystal structure that consists of an infinite framework of tetrahedra linked at corners, that can propagate without the tetrahedra distorting. Because they give distortions of the structure with a low energy cost they are the natural soft modes for displacive phase transitions. We consider the normal phase transition sequence in tridymite on cooling, HP LHP ..., as a successive condensation of rigid unit modes acting as soft modes. Some of the low-temperature phases (e.g. MX-1) arise as rigid unit mode distortions of the high-temperature structure and do not follow the sequence of phases found at higher temperatures. We are able to account for all the commensurate phases and some of the modulated phase within the framework of the rigid unit mode model.
ISSN:0342-1791
1432-2021
DOI:10.1007/s002690050174