An electron diffraction study of the polymorphs of siO2-tridymite

Withers, Raymond; Thompson, John; Xiao, Yiran; Kirkpatrick, R. James

doi:10.1007/bf00202272

Cited by 42 publications

(19 citation statements)

References 22 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A dramatic demonstration of the existence of lowfrequency phonons in I~-cristobalite that vanish on cooling into the c~-phase was obtained by inelastic neutron spectroscopy with powder samples (Swainson and Dove, 1993a). These results were reproduced by calculations of the vibrational density of states by molecular dynamics simulation (Swainson and Dove, 1995 (Withers et al, 1995). The large circles are reciprocal lattice vectors.…”

mentioning

confidence: 53%

Rigid unit modes in framework silicates

1995

View full text Add to dashboard Cite

We describe a model for framework silicates in which the SiO 4 (and AIO4) tetrahedra are treated as perfectly rigid and freely jointed. From this model we are able to identify low-energy modes of distortion of the structure, which we call Rigid unit modes. These modes can act as soft modes to allow easy distortions at phase transition. We discuss three forces that will operate at a phase transition in conjunction with the candidate soft modes to determine which of the rigid unit modes will actually precipitate a phase transition, and illustrate these ideas by detailed discussions of the phase transitions in quartz, leucite and cristobalite. The model can also be used to estimate the transition temperature, and the theory highlights an important role for the stiffness of the tetrahedra.KEVWORDS: rigid unit modes, phase transitions, quartz, leucite, cristobalite. The paradox of framework aluminosilicatesTHOSE of US who come into Mineralogy from other disciplines are frequently struck by a paradox posed by framework aluminosilicates: despite the fact that silicates are amongst the strongest and most stable of materials, there are a number of intriguing theoretical problems associated with their structural stability. One is that aluminosilicates are riddled with phase transitions. Another is that aluminosilicates have considerable chemical flexibility, as they are able to form solid solutions over wide compositional changes. Aluminosilicates sometimes have unusual thermal expansion properties, and zeolites have a number of industrially important chemical properties. So what is going on with these aluminosilicate frameworks?It turns out that, unlike engineering frameworks, the frameworks built from linkages of perfectly rigid SiO4 and A104 tetrahedra are not themselves perfectly rigid. Instead, they can have some internal degrees of freedom of distortion, which give rise to low-frequency phonon modes that can propagate with no distortions of the tetrahedra. We call these Rigid Unit Modes (RUMs), and their existence provides natural candidates for the soft modes that typically drive displacive phase transitions (Dove et al., , 1995a. The idea has its roots in the geometrical polyhedral tilting models of Megaw (1973) and Hazen and Finger (1982).The existence of RUMs in an aluminosilicate framework structure is more subtle a point than might initially be imagined. Each tetrahedron has 6 degrees of freedom, and each linked corner has three constraint equations that prevent the linkage from splitting. Thus the number of constraints per tetrahedron is 89 (from a sharing between two tetrahedra of the constraints on a single linkage) • 4 (the number of comers of a tetrahedron), which is the same as the number of degrees of freedom: the numbers of constraints and degrees of freedom are exactly matched. However, we have shown elsewhere (Dove et aI., 1992;Giddy et al., 1993) that it is possible for some of these constraints to be degenerate, i.e. not independent. A simple example of a degenerate constraint is shown in F...

show abstract

mentioning

confidence: 53%

Rigid unit modes in framework silicates

1995

View full text Add to dashboard Cite

show abstract

“…Based on TEM results, Hezel et al (2003) excluded several possible SiO 2 -polymorphs. A recent TEM-study of the silica phase yielding the unknown spectrum (Hezel, 2003) provided evidence for at least one sub-polymorph of tridymite (this mineral has a total of eight sub-polymorphs; Withers et al, 1994;Pride and Dove, 1998). Further work to unambiguously identify the unknown SiO 2 -phases is in progress.…”

Section: Sio 2 -Polymorphismmentioning

confidence: 95%

Origin of SiO2-rich components in ordinary chondrites

Hezel

Palme

Nasdala

et al. 2006

Geochimica et Cosmochimica Acta

View full text Add to dashboard Cite

“…This is because the eutectic reaction between alkali metals and SiO 2 element occurs during burning, and the carbon remains in the melt SiO 2 and causes the crystallization of rice husk ash at lower temperature [130][131][132][133].…”

Section: (Figure 13)mentioning

confidence: 99%