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Topological versus chemical ordering in network glasses at intermediate and extended length scales

Abstract

Atomic ordering in network glasses on length scales longer than nearest-neighbour length scales has long been a source of controversy1,2,3,4,5,6. Detailed experimental information is therefore necessary to understand both the network properties and the fundamentals of glass formation. Here we address the problem by investigating topological and chemical ordering in structurally disordered AX2 systems by applying the method of isotopic substitution in neutron diffraction to glassy ZnCl2. This system may be regarded as a prototypical ionic network forming glass, provided that ion polarization effects are taken into account7, and has thus been the focus of much attention8,9,10,11,12,13,14. By experiment, we show that both the topological and chemical ordering are described by two length scales at distances greater than nearest-neighbour length scales. One of these is associated with the intermediate range, as manifested by the appearance in the measured diffraction patterns of a first sharp diffraction peak at 1.09(3) Å-1; the other is associated with an extended range, which shows ordering in the glass out to 62(4) Å. We also find that these general features are characteristic of glassy GeSe2, a prototypical covalently bonded network material15,16. The results therefore offer structural insight into those length scales that determine many important aspects of supercooled liquid and glass phenomenology11.

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Figure 1: An illustration of topological versus chemical ordering in disordered network systems.
Figure 2: The measured Bhatia–Thornton partial structure factors for glassy ZnCl2 and GeSe2.
Figure 3: The measured partial pair distribution functions for glassy ZnCl2 and GeSe2.
Figure 4: Extended range ordering is found for glassy ZnCl2 and GeSe2.

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Acknowledgements

We thank M. Skeffington for permission to use his photo in Fig. 1a, R. L. McGreevy for assistance with the isotopes, P. Palleau for help with the diffraction experiment, A. C. Barnes for discussions on the SVD method, A. C. Hannon for a powder diffraction pattern on the polycrystalline phase of ZnCl2, A. C. Wright for providing his data for glassy ZnCl2, P. Buchanan and A. K. Soper for help with an earlier experiment at ISIS, R. Evans, P. A. Madden, C. Massobrio and M. Wilson for discussions, and the EPSRC and Alliance Programme for financial support. P.E.M. made the samples, G.J.C. prepared the diffractometer and R.A.M. made the data analysis. P.S.S. conceived the experiment and was involved in all aspects of the work.

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Correspondence to Philip S. Salmon.

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Supplementary information

Supplementary Figure S1

The measured total structure factors for glassy ZnCl2 at 25(1) °C. (DOC 301 kb)

Supplementary Figure S2

The measured total pair distribution functions for glassy ZnCl2. (DOC 83 kb)

Supplementary Figure S3

The measured Faber–Ziman partial structure factors for glassy ZnCl2. (DOC 112 kb)

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Salmon, P., Martin, R., Mason, P. et al. Topological versus chemical ordering in network glasses at intermediate and extended length scales. Nature 435, 75–78 (2005). https://doi.org/10.1038/nature03475

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