The method of in-situ high-pressure neutron diffraction was used to measure the structural transformations that take place upon cold-compression of several network-forming amorphous materials. The chalcogenide glasses GeSe2, GeSe4, and As2Se3, and the silicate glass CaSiO3, were investigated using a Paris-Edinburgh press to provide compression. Where possible, the neutron diffraction results were compared to experimental results and molecular dynamics (MD) simulations provided by other research groups.
Amorphous GeSe2 was studied at pressures up to 16.1 GPa using a combination
of neutron diffraction, neutron diffraction with isotope substitution (NDIS), and first-principles molecular dynamics (FPMD) simulations. It was found that the network transformations occurred in two stages. In the first stage up to ∼ 8 GPa, the structure re-arranged on an intermediate length scale by re-organising corner and edge-sharing GeSe4 tetrahedra. Above 8 GPa, both 5- and 6-fold coordinated Ge atoms began to form as the mean nearest-neighbour coordination number n and mean nearest-neighbour bond distance r both increased. A disagreement between the neutron diffraction and
FPMD results above 8.5 GPa is attributed to the presence of an energy barrier. This barrier inhibits structural rearrangement in a cold-compression diffraction experiment, but allows them to occur via a high-temperature annealing stage in the simulations.
Amorphous GeSe4 was studied at pressures up to 14.4 GPa using a combination of neutron diffraction and FPMD. The nearest-neighbour coordination environment was found to vary little across the measured pressure range, but structural transformations took place on an intermediate length scale as seen by the pressure-dependence of the second nearest-neighbour distance in the neutron diffraction results. The new experimental results are in accord with FPMD results and with those obtained from a study using x-ray diffraction (XRD). There are, however, major inconsistencies with the results obtained from a different study in which XRD was combined with empirical potential structure refinement (EPSR). It is hypothesised that this disagreement is due to the difficulty of modelling XRD results with EPSR for glasses in the Ge-Se system,
where the x-ray atomic form factors of Ge and Se are similar. The reduced-density ρ/ρ0 dependence of the results was compared to that obtained for amorphous GeSe2, where ρ is the atomic number density at pressure and ρ0 is the atomic number density at ambient pressure. It was found that for both materials the local coordination environment does not change for ρ/ρ0 < 1.6.
Amorphous As2Se3 was studied at pressures up to 14.4 GPa using a mixture of neutron diffraction, NDIS, and FPMD. At the total structure factor level, no change was observed to the nearest-neighbour coordination environment. The NDIS results do, however, suggest a change to the nearest-neighbour coordination environment beginning at 6 GPa. The results were compared to those found for two crystalline polymorphs of As2Se3, one prepared at ambient pressure and the other recovered to ambient conditions from high-pressure and -temperature. The differences between the coordination environments of these crystalline polymorphs pointed to some possible densification mechanisms in the glass. Ambient-pressure NDIS was also used to measure the full set of partial pair-distribution functions.
Amorphous CaSiO3 was studied at pressures up to 17.5 GPa using neutron diffraction. The Si-O coordination number started to increase beyond a threshold pressure of 13 GPa, as compared to 15 GPa for amorphous SiO2. The results were used to test the validity of two sets of MD simulations that used different interatomic potentials and thermal processes for producing the glass. The results were found to agree with the MD simulations that used a cold-compression protocol.
|Date of Award||3 Jul 2015|
|Supervisor||Philip Salmon (Supervisor)|