An investigation of the structure of disordered materials by using neutron diffraction

The structure of several semiconducting, metallic and ionic disordered materials was investigated using neutron diffraction and the results were compared with those obtained from recent ab initio molecular dynamics methods. The method of isotopic substitution was applied to measure the full set of partial structure factors, Sαβ(Q), for the liquid semiconductor GeSe and the covalent network glass GeSe2. Their short range ordering and for GeSe2 also the intermediate range ordering were identified and a substantial number of 'defects' such as homopolar bonds were detected in both systems. Further, the structure of liquid GeSe2 with increasing temperature was studied at the total structure factor level. Also, changes in the topology of GexSe1-x glasses in the range 0 ≤ x ≤ 0.4 were observed and investigated by measuring the total structure factors. As far as possible our results were compared with those from molecular dynamics studies. The Ge-Se system was found to serve as a sensitive test-system for these studies, giving an insight into the strengths and limitations of them. For instance, problems are found in the region of the homopolar bonds for gGeGe(r) and for the first sharp diffraction peak in the Bhatia-Thornton concentration-concentration structure factor that could not be reproduced. The structure of liquid lithium was measured and particular attention was paid to the inelasticity and resolution function corrections. The ion-ion and ion-valence electron partial structure factors were obtained and found to be in good agreement with ab initio molecular dynamics studies. The method of first order difference functions in neutron diffraction in combination with H/D substitution was applied to 2 molal solutions of Cu(ClO4)2 in perchloric acid to measure the Cu-H and, to a first order approximation, the Cu-O partial structure factor. A (4 + 1) distortion of the hydration shell around the Cu2+ ion was measured. (author)