- Rigif Unit Modes in Molecular Dynamics Simulation
of Quartz and The Incommensurate Phase Transition
Frank Stefan Tautz, Volker Heine, Martin T. Dove, and Xiaojie Chen
Phys. Chem. Minerals, vol. 18, 326 (1991)
A molecular dynamics simulation of quartz at different temperatures
both in the alpha and in the beta phase has been conducted.
The alpha-beta phase transition could be observed. A phonon
analysis of the beta-phase confirms and rounds
out in a quantitative way the
origin of the incommensurate (ic) modulated phase.
In particular it traces the optic soft mode at Gamma becoming (to
a good approximation) a so-called rigid unit mode (RUM) at
q neq 0, and elucidates its coupling to the
transverse acoustic mode which precipitates the incommensurate
transition. This success underpins and illuminates the concept
of RUMs and their role in structural phase transitions.
- Simplicity in complexity: Structural phase transitions in
aluminosilicate framework structures
Martin T Dove, X Chen, Andrew P Giddy,
enton D Hammonds and Volker Heine
To be published
Displacive phase transitions are common in complex aluminosilicate
framework structures. We show that they can be treated in terms of a simple
model with semi-rigid units such as SiO$_4$ tetrahedra freely joined at
shared oxygen atoms which can undergo displacements and rotations in the
form of Rigid Unit Modes. Five quantitative tests of the model are given,
and it is shown to explain qualitatively a surprisingly wide range
of observed effects.