I Farnan

M T Dove

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Structure and dynamics of silicate glasses and melts
I. FARNAN* AND M. T. DOVE
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
A one-day meeting was held in the Department of
Earth Sciences, University of Cambridge, on the
theme of the ‘Structure and Dynamics of Silicate
Glasses and Melts’. The meeting was sponsored
by the Mineral Physics group of the Mineralogical
Society, and was attended by ~80 people. The
objective of the meeting was to bring together
experts from the glass community with members
of the mineralogical community who are inter-
ested in glasses and melts. The scope of the talks
considered structural details over length scales
from nearest-neighbour coordinations up to
1 ­ 2 nm, and dynamics in both the slow
relaxational and fast vibrational ranges. The
talks also covered both experimental (particularly
neutron scattering and NMR) and computer
simulation methods.
The meeting was organized into three groups of
themes, namely structure and dynamics in glasses,
structure and dynamics in melts, and vibrational
dynamics in glasses.
The opening talk was by Neville Greaves
(Aberystwyth), with the title ‘Structure and ionic
transport in disordered silicates’ (Greaves, 2000).
This was a wide-ranging talk which addressed
issues that have puzzled glass scientists for some
time, namely, ionic transport in silicate glasses
and the mixed alkali effect. The way in which the
microstructure of a glass may be extrapolated
from an existing knowledge of local structure was
described (Greaves et al., 1997). This in uence of
microstructure on ion dynamics and also on glass
fracture raised some fundamental questions about
glass structure. It was noted that using wide-angle
X-ray scattering of glasses and photo-electron
spectroscopy of freshly cleaved glass surfaces
(Baker et al., 1995), experimental evidence of a
microsegregation of modi er cations was being
established.
The second talk continued the theme of
structure. Phil Gaskell (Cambridge, Physics)
presented recent ideas on the ‘Relationships
between the medium-range structures of glasses
and crystals’ (Gaskell, 2000), something that has
been a topic of long-standing debate which has
been sharpening up over the past few years. The
talk highlighted the fact that the scattering vector
of the  rst sharp peak in diffraction patterns of
glasses is often close to that from the corre-
sponding crystal, as in cristobalite for example.
This suggests some similarities between the
structures of glasses and crystals over some
length scale, which were explored in the talk
(Gaskell and Wallis, 1996; Gaskell, 1998, 2000).
The theme of the comparison between amor-
phous and crystalline materials was continued by
David Keen (Rutherford Appleton Laboratory),
with a talk entitled ‘Comparing the local
structures of amorphous and crystalline poly-
morphs of silica’ (Keen and Dove, 2000). This
described work on amorphous silica and some of
the crystalline phases using Reverse Monte Carlo
analysis of neutron diffraction data. It was noted
that the amorphous silica shares many structural
features with the high-temperature phases of
tridymite and cristobalite over a length scale of
up to 10 AÊ , but the similarities do not extend to
quartz or to the low-temperature phases (Keen,
1997; Keen and Dove, 1999, 2000).
Georges Calas (Paris) spoke on ‘Local environ-
ment around transition elements in synthetic and
natural glasses’ (Galoisy et al., 2000). The results
presented were based on extended X-ray absorp-
tion (EXAFS) and neutron scattering with isotopic
substitution (NSIS) of Ni and Ti in silicate glasses
(Cormier et al., 1999; Cormier et al., 1997). The
occurrence of ‘unusual’  ve-fold coordination for
these transition elements was described. In
addition, considerable cation­ cation correlations
were reported of transition metal cations in corner
or edge-linked polyhedra forming two-dimen-
sional domains of cations (Cormier et al., 1996)
consistent with the ideas presented by Gaskell
earlier (Gaskell, 2000).
Al ex Han non (Ru the rfo rd App le ton
Laboratory) gave another perspective on structure
Mineralogical Magazine, June 2000, Vol. 64(3), pp. 373–376
# 1998 The Mineralogical Society
* e:mail: ifarnan@esc.cam.ac.uk
with his talk ‘Three-coordinated oxygens in some
network glasses’, again using neutron diffraction
data, but this time looking at the structure from
the perspective of the oxygen atoms rather than
that of the cations.
The second theme of melts was introduced by
Ian Farnan (Cambridge, Earth Sciences) in his
talk on ‘Anionic structure and dynamics in silicate
glasses and melts’, which continued with the view
of a structure from the perspective of oxygen with
the additional aspect of the dynamics. This talk
gave a comparison between the shear relaxation
times of silicate melts at high temperature and the
nutation effects in 17O NMR. This provides
evidence for a divergence of oxygen translational
and rotational correlation times. A similar (but at
much lower frequency) decoupling of motional
modes, previously detected by 29Si NMR (Farnan
and Stebbins, 1994), close to the glass transition
can also be observed by simulation of 17O NMR
exchange spectra. It was pointed out that the loss
of the primary relaxation mechanism in silicate
melts in this divergent way is responsible for the
formation of the glass and detailed characteriza-
tion of the time scale of oxygen rearrangements is
pushing forward understanding of this process.
Simon Kohn (Bristol) gave a talk titled ‘Recent
advances in understanding the dissolution
mechanisms of water in silicate melts and
glasses’ (Kohn, 2000). This talk concerned the
quest for a microscopic interpretation of the
dramatic effects on the viscosity and other
transport properties of silicate melts containing
dissolved water (Kohn et al., 1989). In particular
the water dissolution mechanism as a function of
composition was addressed. It was noted that
different dissolution mechanisms existed for
framework aluminosilicate melts and alumi-
nium-free melts. The speciation of water into
H2O and OH
­ entities in the melt in either system
was also noted to be highly temperature
dependent.
Sabyasachi Sen (Aberystwyth) gave the last
talk in the session on melts on the theme
‘Temperature-dependent structural and dynamical
changes in borate, borosilicate and boroaluminate
liquids: high-resolution and high-temperature
NMR studies’. This talk brought together the
connection between the con gurational entropy of
a melt, local structural rearrangement and
transport processes. An impressive array of
NMR results on borate, borosilicate and boroalu-
minate melts and glasses revealed that local
species redistribution could explain changes in
the con gurational entropy of low-alkali borate
melts which exhibit ‘strong’ behaviour. However,
sources of con gurational entropy other than local
rearrangement would be required to explain the
more fragile borosilicate systems (Sen et al.,
1998). High-temperature 29Si and 11B NMR
experiments on borosilicate melts showed that
close to the glass transition there was a divergence
in the rates of motion of the network-forming
cations of B and Si. This is indicative of
relaxation and  ow taking place over longer
distance scales (Stebbins and Sen, 1998) and the
presence of incipient phase separation.
The  nal session considered dynamics in the
vibrational time scale. This subject was intro-
duced by Stephen Elliott (Cambridge, Chemistry),
with a talk entitled ‘Low-frequency vibrational
excitations in vitreous silica: vibrational localiza-
tion, Ioffe-Regel limit, the Boson peak and all
that’. This wide-ranging talk presented results
from molecular dynamics simulations, which
were aimed at understanding some of the issues
associated with the lower-frequency range of the
vibrational dynamics. In particular, the effects of
structural disorder on the propagation of sound
waves was explored, and how the nature of
vibrations changes when the wavelength becomes
comparable to interatomic distances (Taraskin
and Elliott, 1998a,b, 1999a,b).
Mark Harris (Rutherford Appleton Laboratory)
presented the experimental perspective in his talk
on ‘Floppy modes in crystalline and amorphous
silicates by inelastic neutron scattering’ (Harris et
al., 2000). This talk covered both low-energy
vibrations and the origin of the Boson peak at
~5 meV, and showed how many features in the
low-energy vibrational spectra of amorphous
silica and silicates are also seen in inelastic
neutron scattering data from polycrystalline
materials (Dove et al., 1997; Harris et al.,
1999), which reiterates the similarities between
amorphous and crystalline materials discussed
earlier in the talks on structure (Gaskell, 2000;
Keen and Dove, 2000).
The  nal talk was given by Martin Dove
(Cambridge, Earth Sciences), who presented some
theoretical ideas and computer simulations in his
talked entitled ‘Amorphous silica from the Rigid
Unit Mode approach’ (Dove et al., 2000). The
main point was that the similarities in structure
and dynamics of crystalline and amorphous
phases can be understood from the perspective
of the Rigid Unit Mode model. The work has also
shown how amorphous silica can support
I.FARNAN AND M.T. DOVE
374
reversible large-amplitude changes in the struc-
ture, which may be identi ed with tunnelling
modes (Hammonds et al., 1996; Dove et al., 1998,
1999; Trachenko et al., 1998).
In conclusion, a number of important points in
the study of amorphous silicates were aired at the
meeting. Amongst these was the fact that the
similarities between amorphous and crystalline
silicates extends to both structure and vibrational
dynamics. The complementarity of experimental
and computational methods also came across as a
strong point, and in particular the use of
techniques (neutron scattering, NMR, molecular
dynamics simulations) that have only recently
begun to  ourish within the mineralogical
community.
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STRUCTURE AND DYNAMICS OF SILICATE GLASSES AND MELTS
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[Manuscript received 5 November 1999 ]
I.FARNAN AND M.T. DOVE
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Structure and dynamics of silicate glasses and melts

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Structure and dynamics of silicate glasses and melts

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