| Summary: | The method of neutron diffraction with isotopic or isomorphic substitution (NDIS) was employed to investigate the atomic-scale structure of several disordered multi-component materials, thus reducing the complexity of correlations associated with a single diffraction pattern. The solvation of the Cl ion in a 5 molal solution of NaCl in D2O was investigated at temperatures and pressures up to 150 C and 33.8 kbar, respectively, thus extending the state conditions for studying the structure of this geological fluid. Changes to the structure with increasing temperature at 0.1 kbar may be attributed to a reduction in the dielectric permittivity, and the structural variation with increasing pressure at 150 C reflects changes in the structure of water to a more simple-fluid like phase. NDIS with Se-isotopes was used to help untangle the composition-dependent structure of As-Se glasses. The results for As0.30Se0.70 and As0.35Se0.65 favour the formation of a chemically ordered over a chemically disordered network, while the results for As0.40Se0.60 reveal broken chemical order through the appearance of homopolar bonds. The work shows a need to improve previous structural models that were produced by using both the reverse Monte Carlo and first-principles molecular-dynamics methods. Rare-earth clustering in the aluminosilicate glass (R2O3)0.2(Al2O3)0.2(SiO2)0.6 was investigated via NDIS, using Nd and Pr as an isomorphic pair. The results indicate a network structure based on SiO4 tetrahedra along with AlO4 and AlO5 units in the approximate ratio of 4:1. Each rare-earth (R) ion has, on average, 7.3(2) nearest-neighbour oxygen atoms at a distance of rRO = 2.43(2) A, and the R-R nearest neigh-bour distance is 3.9(4) A. The results are discussed by reference to the structure of a rare-earth glass of the same composition, but where Nd/Pr is replaced by a smaller rare-earth ion pair such as Dy or Ho.
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