NEWS & VIEWS

842� nature materials | VOL 7 | NOVEMBER 2008 | www.nature.com/naturematerials

Giancarlo Ruoccois in the Physics Department at the ‘Sapienza’, University of Rome, 00185 Rome, Italy, and in the INFM-CNR Research Centre on Soft Matter, 00185 Rome, Italy.

e-mail: giancarlo.ruocco@roma1.infn.it

Amorphous solids such as glasses are defined by the lack of long-range structural order that gives

rise to a complex phenomenology in their vibrational properties. The thermal motion of atoms and molecules in these glasses cannot be simply described in terms of the wave-like oscillations known to solid-state physicists as phonons. From a macroscopic point of view, this complexity is reflected in anomalies that appear in the thermal and transport properties of glasses as well as in their density of states, all of which distinguish amorphous materials from their crystalline counterparts. Writing on page 870 of this issue, Hiroshi Shintani and Hajime Tanaka1 look at the origin of one of the universal features at the heart of the anomalous properties of glasses, the boson peak.

Among the thermal anomalies of glasses, the most striking are the temperature dependences of specific heat and thermal conductivity. The thermal conductivity of non-crystalline solids is markedly different from that of a crystal. In the low-temperature region it is several orders of magnitude smaller and shows a different power-law temperature dependence. Furthermore, the thermal conductivity has a plateau in the 1–10 K temperature region2–4. Remarkably, the thermal conductivity is not materials-dependent and almost all glasses show the same behaviour, not just qualitatively but quantitatively. This poses the intriguing question: why do all glasses have the same thermal conductivity curve at low temperatures? The plot thickens further when we consider another thermal property of glasses, the specific heat.

The temperature dependence of specific heat also shows features common to all glasses. When plotting the specific heat divided by the temperature cubed (C/T 3),

in a way that should lead to a constant line conforming to the ‘Debye behaviour’ found in crystals, one observes a bump for which the specific heat is increased. Although the magnitude of this excess changes for different amorphous solids, this anomalous hump is commonly seen at temperatures between 5 and 10 K. Eventually, the excess specific heat can be traced back to the presence of additional phonon states that do not exist for materials conforming to the Debye law.

These extra vibrational modes have been observed in a number of glasses5,6. In plotting the number of phonon states against their frequency, again one observes an excess with respect to the expectation from the Debye law. This bump in the phonon spectrum is referred to as the ‘boson peak’ and is directly related to the anomalous excess in specific heat. The name originates from the observation that in temperature-dependent experiments the boson peak scales as expected from a system obeying the Bose–Einstein population statistic.

Although the relation between the boson peak and the specific heat is rather trivial, the link between the plateau in the temperature dependence of the thermal conductivity and the boson peak has only recently been fully clarified7,8. However, having associated the thermal anomalies in glasses to the boson peak, the key questions remain the same: what is the origin of the boson peak, and what kind of phonons contribute to it? Are these phonons that are localized or are these extended and propagating states9,10? Are they longitudinal or transverse11–13? And, even more importantly, what is the origin of this universality?

The answers to these questions could have important consequences. It is, indeed, easy to see how useful this understanding would be for the design of new glassy materials with desired thermal properties. Controlling the specific heat means one can control the capability of a material to store thermal energy, and controlling the thermal conductivity allows the design of improved thermal insulators. Ultimately, any attempt to tailor the thermal properties of glasses requires a detailed understanding of the boson peak.

Shintani and Tanaka have now made a decisive step forward in answering the questions on the character of the phonon modes giving rise to the boson peak. Using numerical simulation techniques, they have studied the atomic motions in a class of model glasses where, by changing one of the parameters that controls the interatomic forces, they can modify the glass properties, and in particular, the boson peak’s position and intensity. By studying the vibration modes of these model glasses, Shintani and Tanaka have been able to associate the excess phonon modes unambiguously to transverse-like vibrations (vibrations in which atomic planes slide with respect to each other). Unlike longitudinal vibrations, which correspond to compression and rarefaction of a material and can be measured in a number of experiments, transverse phonons cannot be studied experimentally, and numerical simulations are mandatory.

Amorphous solids show intriguing universal behaviour whose origins often remain poorly understood. One of these features, the boson peak, is now shown to be directly linked to transverse vibrations.

GlAssEs

when disorder helps

Figure 1 typical structure of a glassy state as modelled by shintani and tanaka1. Red and green particles have high and low medium-range crystalline order, respectively, and blue particles are locally favoured structures of five-fold symmetry that are associated with the boson peak.

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Hint

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ime

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© 2008 Macmillan Publishers Limited. All rights reserved.

NEWS & VIEWS

nature materials | VOL 7 | NOVEMBER 2008 | www.nature.com/naturematerials 843

As found by Shintani and Tanaka, the transverse vibrations that give rise to the boson peak tend to be particularly strong around certain local atomic arrangements in the glass. Because transverse modes are much more sensitive to the lack of order than longitudinal ones, Shintani and Tanaka suggest that these transverse excitations become stuck around the disordered structures, and their frequency spectrum is centred on the frequency of the boson peak. Longitudinal modes, on the other hand, are much less affected by the disorder and continue to propagate at frequencies beyond the boson peak.

This picture considerably expands recent theoretical results11–14 and sheds light on the longstanding issue of the origin of the boson peak. Indeed, although certain aspects need further clarification, a complete understanding of the boson peak is now emerging. This could pave the way to designing new materials for the storage of thermal energy or for thermal insulation.References1. Shintani, H. & Tanaka, H. Nature Mater. 7, 870–877 (2008).2. Zeller, R. C. & Pohl, R. O. Phys. Rev. B 4, 2029–2041 (1971).3. Freeman, J. J. & Anderson, A. C. Phys. Rev. B

34, 5684–5690 (1986).4. Pohl, R. O., Liu, X. & Thompson, E. Rev. Mod. Phys.

74, 991–1013 (2002).

5. Sokolov, A. P., Kisliuk, A., Soltwisch, M. & Quitmann, D. Phys. Rev. Lett. 69, 1540–1543 (1992).6. Buchenau, U., Wischnewski, A., Ohl, M. & Fabiani, E.

J. Phys. Condens. Matter 19, 205106 (2007).7. Allen, P. B. & Feldman, J. L. Phys. Rev. B

48, 12581–12588 (1993).8. Schirmacher, W. Europhys. Lett. 73, 892–898 (2006).9. Matic, A., Engberg, D., Masciovecchio, C. & Börjesson, L.

Phys. Rev. Lett. 86, 3803–3806 (2001).10. Ruocco, G. & Sette, F. J. Phys. Condens. Matter

13, 9141–9164 (2001).11. Schirmacher, W., Diezemann, G. & Ganter, C. Phys. Rev. Lett.

81, 136–139 (1998).12. Taraskin, S. N., Loh, Y. L., Natarajan, G. & Elliott, S. R.

Phys. Rev. Lett. 86, 1255–1258 (2001).13. Grigera, T. S., Martín-Mayor, V., Parisi, G. & Verrocchio, P.

Nature 422, 289–292 (2003).14. Schirmacher, W., Ruocco, G. & Scopigno, T. Phys. Rev. Lett.

98, 025501 (2007).

Paul F. mcmillan is in the Department of Chemistry, Christopher Ingold Laboratory and Materials Chemistry Centre, University College London, 20 Gordon Street, London WC1H 0AJ, UK.

e-mail: p.f.mcmillan@ucl.ac.uk

The structures and properties of molten ceramics are of considerable interest, especially for developing their

technological applications. For example, systems based on Al2O3 mixed with metal oxides are studied because they give rise to optical materials including laser hosts and important families of structural ceramics. But experimental studies are challenging because of the very high temperatures involved (often exceeding 2,000 °C), and because the liquids are so corrosive. An elegant solution is provided by containerless levitation in which the ceramic sample is floated on a cushion of air provided by a gas stream passing through a convergent–divergent nozzle and then melted using an infrared laser (Fig. 1). The levitated liquid can then be studied by liquid NMR spectroscopy and X-ray or neutron scattering1.

This was the route followed by Neville Greaves and colleagues for their study of yttrium aluminate liquids as they were cooled from 2,500 K to around 1,500 K, into the deeply supercooled regime, as recently reported in Science2. In particular, they used X-ray scattering at the Daresbury SRS synchrotron facility

(now closed since August 2008)3 and the Advanced Photon Source (Argonne National Laboratory) to study the local structure and medium-range order in the levitated molten ceramics. They found that as the temperature of the supercooled liquids decreased, there was an unexpected rise followed by a maximum in the intensity of the small-angle scattering signal (Fig. 2). The authors assign this unusual

behaviour to the onset of a liquid–liquid phase transition due to fluctuations in the density rather than chemical composition occurring within the liquid state4.

This is a remarkable observation. It confirms previous indications from optical microscopy studies and calorimetry on quenched glasses that there is a ‘polyamorphic’ liquid–liquid phase transition in Y2O3–Al2O3 liquids5,6. The suggestion that such density-driven transitions might occur in liquids at constant chemical composition first arose from observations of unusual negative melting slopes (dTm/dP) and maxima in melting curves that were observed in high-pressure and high-temperature (high-P,T) experiments for various elements and simple compounds. Thermodynamic analysis using a ‘two-state’ model for the liquid containing low- and high-density structural configurations indicated that a critical point should develop at low temperature, followed by a line of first-order liquid–liquid phase transitions analogous to those that occur for crystalline solids. This was a surprising prediction because liquids were thought to have rapidly changing local structures that underwent constant thermal averaging. However, liquid phase transitions driven by gradients in the chemical potential resulting in compositional unmixing were well known. Observation of a phase transition driven by density (ρ = 1/V) or entropy (S) would thus complete the thermodynamic trilogy of the

Refractory ceramic liquids studied by containerless levitation and synchrotron X-ray scattering reveal an unusual density-driven liquid–liquid phase transition.

cERAmic mAtERiAls

levitating liquids

Figure 1 A levitated ceramic liquid sample mounted inside an aerodynamic levitator at the Advanced Photon source for synchrotron X-ray scattering experiments. the conical levitator directing the air flow is seen at the bottom and the sample showing incandescent thermal emission is heated by a co2 laser. the levitator is mounted on the synchrotron beamline, permitting X-ray small-angle and wide-angle scattering experiments. Photo courtesy of m. Wilding, Aberystwyth University.

© 2008 Macmillan Publishers Limited. All rights reserved.