Local structure of the vacancy disordered fluorite Yb 3 TaO 7 from neutron total scattering

The local structure of the defect, vacancy disordered fluorite Yb3TaO7 has been investigated using neutron total scattering methods. The average structure of Yb3TaO7 is well described in space group Fmm, with unit cell constant a = 5.1872(1) Å, in which Yb and Ta occupy, randomly, the single cation site in a fluorite structure and there is a vacancy concentration of approximately ⅛ on the anion site. Examination of the neutron diffraction data shows many features in the background which can be ascribed to local order of Yb, Ta, and O atoms. The pair distribution function, G(r), was analyzed using both crystallographic models and Reverse Monte Carlo (RMC) methods. From both perspectives the local structure resembles very strongly the cation and vacancy ordered model expected by extrapolation from the structure of Ho3TaO7, space group C2221. As well, this local structure model provides a superior fit to the G(r) out to 30 Å compared to the average structure model. This result may have implications for the understanding of phenomena such as catalytic activity which are reported to depend on order/disorder structural transformations in the series, RE3TaO7 and RE3NbO7 where RE is a rare earth element.