Cation order/disorder and local structures in alkaline earth pyrochlores

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International Union of Crystallography (IUCr)
Materials that form the A2-yB2O7-x pyrochlore structure have various applications including use as catalysts, fuel cells, piezoelectrics, ferroelectric devices and ferro-magnets; have a wide range of electrical and ionic conductivities, including metallic, semi and super; can be used in nuclear-waste immobilisation due to radiation toughness, and have ion exchange properties. The degree of disorder of the A-cation is important and may play the major role in the effectiveness of these materials ion exchange properties; and contribute to the high relative permittivities displayed by (Bi,Zn)2(Zn,Nb)2O7. Previous Neutron and X-ray diffraction studies suggest that the disorder involves displacement of the A-cation along the six <112> or <110> directions. Our observation of diffuse scattering in electron diffraction patterns of CsTi0.5W1.5O6 pyrochlores suggests there are strong local correlations among the disordered ions. Movement of O at 48f away from 0.375 reduces the interaction between the two networks and so may increase the amount of disorder of the A-cation. The structures of the defect pyrochlores AAl0.33W1.67O6 where A= K, Rb or Cs have been investigated using an array of advanced structural probes; X-ray and neutron diffraction methods, NMR spectroscopy as well as the ab-initio modeling using VASP. The structures do not show a simple correlation between the radius of the A-type cation and the cubic lattice parameter. Our structural studies suggest that this may reflect the degree of local disorder of the A-cation. The results of these studies will be presented in this presentation. © 2008 International Union of Crystallography
X-ray diffraction, Neutron diffraction, Neutron diffractometers, Crystallography, Oxides, Cations, Pyrochlore
Thorogood, G. J., Kennedy, B. J., Peterson, V. K., Elcombe, M. M., Kearley, G. J., & Hanna, J. V. (2008). Cation order/disorder and local structures in alkaline earth pyrochlores. Presentation at the XXI IUCr Congress ( International Union of Crystallography), Osaka, Japan, 23 to 31 August 2008. In Acta Crystallographica Section A: Foundations and Advances, 64(a1), C512. doi:10.1107/S0108767308083542