Local structure in disordered materials studied by neutron scattering and RMC modelling

Sammanfattning: Different disordered materials are becoming more and more important in various technological applications. There is a need to improve our detailed understanding of the local structure of such materials and how the atomic structure is related to macroscopic physical properties. Neutron diffraction in combination with Reverse Monte Carlo (RMC) modelling has proved to be a very useful method for studying disordered materials. With RMC it is possible to produce three dimensional structural models that agree quantitatively with the total neutron structure factor and with other data (e.g. X-ray and EXAFS) if available. A number of different disordered materials were studied by this method:Orientational ordering of the linear cyanide ions in potassium cyanide was studied in two different crystallographic phases.The mechanism of ionic conduction in the fast ion conductors Li2SO4 and LiNaSO4 was investigated and found to be a mixture of the two already existing (and competing) ideas; 'paddle wheel' and 'percolation'.Local lattice distortions characterised in terms of 'short' and 'long' Cu(2)-O(4) bonds were found in the high Tc, superconductor YBa2Cu3O6.95. From the number of 'short' bonds the charge transfer, the superconducting correlation length and the transition temperature can be obtained. The variation of Tc with oxygen composition and Co doping was also found to be closely related to the number of 'short' bonds.The magnetic structures of the amorphous metallic glasses Dy7Fe3 and Fe0.91Zr0.09 were studied. The former is a speromagnet, where random magnetic anisotropy dominates over the exchange interactions. The FeZr glass is predominantly ferromagnetic, but defects consisting of a small number of magnetic moments that point in the opposite direction to the net magnetisation were identified. The local Fe density around these defects is higher than around 'normal' moments. It is suggested that antiferromagnetic 'clusters' are formed naturally as a result of the local atomic density fluctuations within such an amorphous material.

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