The role of structural disorder in crystalline, glassy and liquid materials

Abstract

The method of neutron diffraction with isotope substitution (NDIS) was employed to measure the structures of crystalline ice-Ih, amorphous GeSe3 and GeSe4 and a 5 molal solution of NaCl in D2O.

For crystalline ice-Ih, the complete set of partial structure factors was measured at temperatures of 15 K and 123 K using hydrogen/deuterium isotope substitution to help clarify the nature of the proton disorder associated with the hydrogen bond network. The disorder manifests itself in the asymmetric profiles of the intra-molecular H-H peak and first inter molecular O-H peak, in both ice-Ih and low density amorphous ice. The measured partial pair-distribution functions show that each oxygen molecule is hydrogen bonded to two others and indicates that the hydrogen bond is bent, with an average O-H· · · O angle of 160.1(4.4)◦ at 15 K and 177.2(2.4)◦ at 123 K.

In the work on the prototypical network forming glasses GeSe3 and GeSe4, the full set of partial structure factors were measured using the NDIS technique. A comparison of the structures of glassy GeSe2, GeSe3 and GeSe4 shows that whilst GeSe3 and GeSe4 are chemically ordered continuous random network glasses, the intrinsic chemical order of glassy GeSe2 is broken by the existence of Ge-Ge and Se-Se homopolar bonds. There are some discrepancies associated with the pair-correlation functions calculated using first-principles molecular dynamics simulations, particularly with respect to the Ge-Ge correlations, which indicates scope for improvement in these simulations.

In situ high pressure neutron diffraction experiments using Cl isotope subtitution were performed on a 5 molal solution of NaCl in D2O at 150◦C. The O-D bond distances and coordination numbers did not change over the measured pressure range of 0.24 GPa – 3.38 GPa. Site specific information of the Cl− coordination environment was obtained from the difference functions, which showed that as the pressure increases the nearest- neighbour Cl-D coordination number decreases from 7.1(5) to 6.3(5), as the associated Cl-D distance decreases from 2.21(4) ˚A to 2.18(4) ˚A.

Date of Award	4 Apr 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Philip Salmon (Supervisor)