Abstract

The existence of a structural motif or motifs that frustrate periodic order and give rise to good glass-formability has not been experimentally confirmed, despite long-standing hypotheses bolstered by simulation. This lack of empirical evidence reflects the fact that the experimental determination of disordered structures remains a substantial challenge.

It 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-order correlation functions (3- and 4-body) in the material, and are usually averaged out if the diffracted volume samples too many short-range configurations. These higher-order functions are extremely sensitive probes of atomic-range order in the material.

In this talk I will present statistical measurements of angular correlations in electron nano- and x-ray micro- diffraction patterns of metallic glasses and glassy colloids, respectively [2,4]. I will outline developments in the dynamical diffraction theory of disordered structures that enable the interpretation of these angular symmetries in the diffraction plane in terms of the symmetries of the short-range polyhedral clusters in the structure itself. The three-dimensional “bond-orientational order” parameters that were developed by Steinhardt et al. have been an invaluable tool for assessing local order in systems in which particle positions are known [5]. We have developed a set of “projected bond-orientational order” parameters for the transmission 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.

[1] J. M. Ziman, ‘Models of Disorder: The Theoretical Physics of Homogeneously Disordered Systems’, Cambridge University Press, (1979)

[2] A. C. Y. Liu et al., Phys. Rev. Lett. 110, 205505 (2013)

[3] M. M. J. Treacy, J. M. Gibson, L. Fan, D. J. Paterson, I. McNulty Rep. Prog. Phys. 68, 2899 (2005)

[4] P. Wochner et al., Proc. Natl. Acad. Sci. 106, 11511 (2009).


[5] P. J. Steinhardt et al., Phys. Rev. B 28 , 784 (1983). 


[6] A. C. Y. Liu et al., Phys. Rev. Lett. in press (2016)