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SUMMARY:Bond-Orientational Order in Metallic and Colloidal Glasses Measure
 d by Limited-Volume Electron and X-ray Diffraction - Amelia Liu\, Monash C
 entre for Electron Microscopy\, Monash University\, Clayton\, Australia
DTSTART:20160616T130000Z
DTEND:20160616T140000Z
UID:TALK66222@talks.cam.ac.uk
CONTACT:Duncan Johnstone
DESCRIPTION:The existence of a structural motif or motifs that frustrate p
 eriodic order and give rise to good glass-formability has not been experim
 entally confirmed\, despite long-standing hypotheses bolstered by simulati
 on.  This lack of empirical evidence reflects the fact that the experiment
 al determination of disordered structures remains a substantial challenge.
  \n\nIt is well known that the 2-body correlation function measured using 
 standard broad-beam diffraction is not sufficient to discriminate between 
 competing structural models in disordered materials [1].  By limiting the 
 coherence length of the incident radiation to a size comparable to that of
  the short-medium range structural units\, fluctuations in the diffracted 
 intensity may be detected [2\, 3].  These fluctuations relate to higher-or
 der correlation functions (3- and 4-body) in the material\, and are usuall
 y averaged out if the diffracted volume samples too many short-range confi
 gurations.  These higher-order functions are extremely sensitive probes of
  atomic-range order in the material.\n\nIn this talk I will present statis
 tical measurements of angular correlations in electron nano- and x-ray mic
 ro- diffraction patterns of metallic glasses and glassy colloids\, respect
 ively [2\,4].  I will outline developments in the dynamical diffraction th
 eory of disordered structures that enable the interpretation of these angu
 lar symmetries in the diffraction plane in terms of the symmetries of the 
 short-range polyhedral clusters in the structure itself.  The three-dimens
 ional “bond-orientational order” parameters that were developed by Ste
 inhardt et al. have been an invaluable tool for assessing local order in s
 ystems in which particle positions are known [5].  We have developed a set
  of “projected bond-orientational order” parameters for the transmissi
 on diffraction geometry [6].  These can be used to measure proportions of 
 different local structures directly from the average angular correlations 
 in limited-volume diffraction patterns of glasses.\n\n\n[1] J. M. Ziman\, 
 ‘Models of Disorder: The Theoretical Physics of Homogeneously Disordered
  Systems’\, Cambridge University Press\, (1979)\n\n[2] A. C. Y. Liu et a
 l.\, Phys. Rev. Lett. 110\, 205505 (2013)\n\n[3] M. M. J. Treacy\, J. M. G
 ibson\, L. Fan\, D. J. Paterson\, I. McNulty Rep. Prog. Phys. 68\, 2899 (2
 005) \n\n[4] P. Wochner et al.\, Proc. Natl. Acad. Sci. 106\, 11511 (2009)
 .
\n\n[5] P. J. Steinhardt et al.\, Phys. Rev. B 28\, 784 (1983). 
\n\
 n[6] A. C. Y. Liu et al.\, Phys. Rev. Lett. in press (2016)
LOCATION:LT Goldsmith 1 Materials Science and Metallurgy\, Department of
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