Local structure and ionic conductivity in the Zr2Y2O7-Y3NbO7 system

The Zr0.5-0.5xY0.5+0.25xNb0.25xO1.75 solid solution possesses an anion-deficient fluorite structure across the entire 0 <= x <= 1 range. The relationship between the disorder within the crystalline lattice and the preferred anion diffusion mechanism has been studied as a function of x, using impedance spectroscopy measurements of the ionic conductivity (sigma), powder neutron diffraction studies, including analysis of the 'total' scattering to probe the nature of the short-range correlations between ions using reverse Monte Carlo (RMC) modelling, and molecular dynamics (MD) simulations using potentials derived with a strong ab initio basis. The highest total ionic conductivity (sigma = 2.66 x 10(-2) Omega(-1) cm(-1) at 1473 K) is measured for the Zr2Y2O7 (x = 0) end member, with a decrease in s with increasing x, whilst the neutron diffraction studies show an increase in lattice disorder with x. This apparent contradiction can be understood by considering the local structural distortions around the various cation species, as determined from the RMC modelling and MD simulations. The addition of Nb5+ and its stronger Coulomb interaction generates a more disordered local structure and enhances the mobility of some anions. However, the influence of these pentavalent cations is outweighed by the effect of the additional Y3+ cations introduced as x increases, which effectively trap many anions and reduce the overall concentration of the mobile O-2 species.