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pagetitle-crest Polyamorphism

LB Skinner, P Kidkunthod & AC Barnes

Collaborators: M Wilding (Aberystwyth University)

Poly-a-morphism is the amorphous analogue of polymorphism observed in crystals. It is the idea that there can exist multiple, structurally distinct amorphous forms of the same material, between which there is a first order-like phase transition. Polyamorphism and the related amorphous-amorphous transitions is a subject of much debate. Several liquid and glassy materials (including H2O) have been reported to exist in different density states under extremes of temperature and pressure. One such system that we have studied is Y2O3-Al2O3 glasses [1].

Some important unanswered questions that surround polyamorphism and the corresponding liquid-liquid, or amorphous-amorphous phase transitions are:

  • Do true thermodynamic liquid-liquid phase transitions really exist? This has not yet been confirmed beyond reasonable doubt.


  • Do they end in a critical point? Although critical points are predicted, only the transitions have been observed experimentally.


  • Are these transitions more general, or are they limited to a small number of materials? So far polyamorphism has been observed or predicted in tetrahedral local order systems only. Why is this? Does polyamorphism require a density maximum like that seen in water?


  • Do liquid-liquid phase transitions precede or trigger crystallisation in deeply undercooled melts? in several polymeric systems large scale ordering has been observed to precede crystallisation, could this be density fluctuations of a liquid-liquid transition, immediately prior to crystallisation?


  • Why do liquid-liquid transitions occur?? Is density alone the driving force for these transitions, or are additional order parameters required?


  • What physical property changes accompany these transitions? These transitions could have important implications for magmatic liquids, which experience high pressures and temperatures inside this and other planets.


  • Is there any number of possible polyamorphs, or can only two exist in the same material?
glass_yttria_poly

Top - samples produced by levitation. left to right Y2O3 content is 20, 24, 30, 37.5, 41 mol%. Two extreme samples are completetly crystalline the central sample is single phase glass. The 24 and 37.5 samples are both turbid. Bottom - phase contrast optical microscope images, AY37.5 contains inclusions (shown to be crystalline by X-ray diffraaction) whereas the AY24 sample does not contain inclusions. The intresting question is then what is causing the turbity, is it the beginnings of density driven phase separation (i.e. polyamorphism) or composiotnally driven separation, that has been arrested on cooling.

With our work on Aluminate glasses, we aim to, learn about the structural and dynamical changes that accompany polyamorphism, allowing us to contribute towards answering some of these important questions.


pagetitle-crest References

[1] L.B. Skinner, A.C. Barnes, P.S. Salmon & W.A. Crichton, Phase separation, crystallisation and polyamorphism in the Y2O3-Al2O3 system. J. Phys.: Condens. Matter 20, 205103 (2008).