Local structure and ionic conductivity in theZr2Y2O7–Y3NbO7 system

Abstract

The Zr0.5−0.5xY0.5+0.25xNb0.25xO1.75 solid solution possesses an anion-deficient fluorite structure across the entire0≤x≤1 range. The relationship between the disorder within the crystalline lattice andthe preferred anion diffusion mechanism has been studied as a function ofx, using impedance spectroscopy measurements of the ionic conductivity (σ), powder neutron diffraction studies, including analysis of the ‘total’ scatteringto probe the nature of the short-range correlations between ions using reverseMonte Carlo (RMC) modelling, and molecular dynamics (MD) simulations usingpotentials derived with a strong ab initio basis. The highest total ionic conductivity (σ = 2.66 × 10−2 Ω−1 cm−1 at 1473 K) is measuredfor the Zr2Y2O7 (x = 0) end member,with a decrease in σ with increasing x, whilst the neutron diffraction studies show an increase in lattice disorder withx. Thisapparent contradiction can be understood by considering the local structural distortions around thevarious cation species, as determined from the RMC modelling and MD simulations. The additionof Nb5+ and its stronger Coulomb interaction generates a more disordered localstructure and enhances the mobility of some anions. However, the influenceof these pentavalent cations is outweighed by the effect of the additionalY3+ cationsintroduced as x increases, which effectively trap many anions and reduce the overall concentration of the mobileO2− species.