Glass - Back to the Future!

Presenting Author:
Gregory S. Moody

article posted 26 May 2016

Greg Moody graduated from the University of Kent in 2015 with an MPhys in Physics, his final year project focused on a structural and magnetic characterisation of the transition metal oxide KOsO4. He is currently enrolled on a PhD course at the University of Bath, under the supervision of Prof. Philip Salmon. The primary focus of his research is the structure and properties of liquids and glasses under extreme conditions.

Density Driven Structural Transformations in Magnesium Silicate Glass

Gregory S. Moody1, Philip S. Salmon1, Anita Zeidler1, Michela Buscemi1, Annalisa Polidori1,2, Henry E. Fischer2, Craig L. Bull3, Mark Wilson4
1Department of Physics, University of Bath, Bath BA2 7AY, UK
2Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, France
3ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
4Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK

Magnesium silicates (MgO)x(SiO2)1-x (0 ≤ x ≤ 1) have been extensively studied on account of their scientific and technological importance. They form a high proportion of the Earth’s mantle, so that the liquid structure is of importance for understanding magma-related processes [1], and it is possible to smoothly progress from a “strong” to a “fragile” glass-forming material with increasing Mg content [2]. At ambient pressure, the small Mg-O coordination number of ~4.5 means that Mg can act as either a network former or a network modifier, i.e., the structural role of Mg is ambiguous [3, 4].

Here we use neutron diffraction to investigate the pressure-dependent structure of glassy MgSiO3. This composition corresponds to the magnesium-rich end member of the pyroxene silicate mineral series enstatite-ferrosilite (MgSiO3-FeSiO3), where these end members are common rock-forming minerals. The experiments were performed using a Paris-Edinburgh press and either the diffractometer D4c (ILL, Grenoble) at pressures up to ~8 GPa, or the diffractometer PEARL (ISIS, Didcot) at pressures up to ~17.5 GPa. The pressure-induced change to the coordination environment of Mg is quantified, and demonstrates a transition in the role of Mg from a network former to a network modifier. The structural changes are compared to those observed for glassy CaSiO3 and SiO2, and the mechanisms of network collapse are elucidated with the help of molecular dynamics simulations.

[1] Karki B B, Zhang J and Stixrude L 2013 Geophys. Res. Lett. 40 94
[2] Wilding M C, Benmore C J and Weber J K R 2010 Europhys. Lett. 89 26005
[3] Cormier L and Cuello G J 2011 Phys. Rev. B 83 224204
[4] Kohara S et al. 2011 Proc. Natl. Acad. Sci. USA 108 14780