Cation composition effects on oxide conductivity in theZr2Y2O7–Y3NbO7 system

Abstract

Polarizable interaction potentials, parametrized using ab initio electronicstructure calculations, have been used in molecular dynamics simulationsto study the effect of cation composition on the ionic conductivity in theZr2Y2O7–Y3NbO7 system and to link the dynamical properties to the degree of lattice disorder.Across the composition range, this system retains a disordered fluoritecrystal structure and the vacancy concentration is constant. The observedtrends of decreasing conductivity and increasing disorder with increasingNb5+ content were reproduced in simulations with the cations randomly assigned to positions onthe cation sublattice. The trends were traced to the influences of the cation chargesand relative sizes and their effect on vacancy ordering by carrying out additionalcalculations in which, for example, the charges of the cations were equalized. Thesimulations did not, however, reproduce all of the observed properties, particularly forY3NbO7. Its conductivity was significantly overestimated and prominent diffuse scatteringfeatures observed in small area electron diffraction studies were not alwaysreproduced. Consideration of these deficiencies led to a preliminary attempt tocharacterize the consequence of partially ordering the cations on their lattice,which significantly affects the propensity for vacancy ordering. The extentand consequences of cation ordering seem to be much less pronounced on theZr2Y2O7 side of the composition range.