Local structure of the vacancy disordered fluorite Yb3TaO7 from neutron total scattering

Abstract

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.