糖心TV

Disorder in materials can range from the complete lack of long-range order in glasses, to the non-periodic displacement or misplacement of atoms in disordered crystals. Glasses and other amorphous materials, however, do possess short range order, and this can be characterised by an atom-atom pair correlation function, derived experimentally by Fourier transformation of the diffraction pattern over its full range 聳 hence the term 聭total scattering聮. For disordered crystalline materials the unit cells are not all identical and hence, although they exhibit Bragg diffraction, they have structural properties which are not revealed by conventional crystallographic methods. Thus it is becoming increasingly common to apply the pair correlation function method to crystals, as well as glasses. By these means it can be shown that the local structure (short range order) may differ from the long range structure determined solely from the Bragg diffraction peaks. The recently constructed GEneral Materials diffractometer, GEM, at the ISIS neutron source is the most advanced neutron diffractometer for total scattering studies in the world. The talk will discuss the study of the structure of disordered materials by neutron total scattering, using results from GEM as examples. These include: - The evolution of 5-coordinated Ge as an explanation of the germanate anomaly in glasses, - magnetic differences for finding ions in glasses, - rings in borate glasses, - skipping ropes made from cyanides, - the search for oxo groups in a uranium oxide. (See http://www.amorphousstructure.sgthome.co.uk/06esg/index.html for a tutorial on 聯Neutron diffraction as a probe of local structure聰)