Si--O and Al--O bonds are very strong, so that it may come as a surprise
that aluminosilicates are riddled with structural phase transitions,
particularly displacive transitions. For example, quartz (SiO2)
has two extended phases, alpha and beta, and an incommensurate phase within a
small temperature range between them. The feldspar series,
(Na,K)AlSi3O8--CaAl2Si2O8, which is one of the most
important geological materials, contains a number of different displacive
and Al--Si site ordering phase transitions.
These displacive phase transitions arise from the fact that
aluminosilicates have framework structures, consisting of semi-rigid
SiO4 and AlO4 tetrahedra and AlO6 octahedra, loosely jointed by
shared oxygen atoms at a corner or along an edge. The force constants for
deforming these units are typically an order of magnitude stronger than the
weaker forces for rotating one unit about the other at the common corner or
edge. It is therefore a good first approximation to take the units as
completely rigid and the joints as completely free. The second
is to give the units a finite stiffness conveniently characterised by a
single large stiffness constant S (strong) , and to include weak forces
W between the units.
The concept of ``rigid unit phonon modes'' (RUM) is constructed based
on such structural observations. It gives a basis for
understanding why phase transitions are found in many aluminosilicates,
with the RUMs playing the role of the conventional soft mode.
units are so stiff that all these phase transitions would be quite
impossible if they were not driven by unstable RUMs.
Moreover the displacive
instabilities are known to have a large role in driving atomic ordering, so
that our model is relevant to those also. We expect that the basic ideas
discussed here will also be important in aluminosilicate glasses, where the
existence of RUMs (called floppy modes in the context of glasses) has been
demonstrated. Technologically the most important application of these ideas
might relate to the thermal expansion properties of glasses. We also expect
that our ideas will have parallels for phase transitions in non-aluminosilicatecrystal systems.
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