Impact of uniaxial strain and doping on oxygen diffusion in CeO2

Doped ceria is an important electrolyte for solid oxide fuel cell applications. Molecular dynamics simulations have been used to investigate the impact of uniaxial strain along the directions and rare-earth doping (Yb, Er, Ho, Dy, Gd, Sm, Nd, and La) on oxygen diffusion. We introduce a new potential model that is able to describe the thermal expansion and elastic properties of ceria to give excellent agreement with experimental data. We calculate the activation energy of oxygen migration in the temperature range 900-1900K for both unstrained and rare-earth doped ceria systems under tensile strain. Uniaxial strain has a considerable effect in lowering the activation energies of oxygen migration. A more pronounced increase in oxygen diffusivities is predicted at the lower end of the temperature range for all the dopants considered

Conduction and disorder in Y(3)NbO(7) - Zr(2)Y(2)O(7)

The construction of interaction potentials for the Y 0.5+0.25xNb0.25xZr0.5-0.5xO1.75 system, on a purely ab-initio basis, is described. These potentials accurately reproduce experimental data on both the structure and the dynamics of these systems; the computer simulations also reproduce the experimental trend of the conductivity, which decreases as x increases, and of the level of static disorder within the O2- sublattice, which increases with x. A detailed analysis of these phenomena shows that the static disorder in Y 3NbO7 is caused by the high Nb5+ charge and that in this material the conduction is heterogeneous, i.e. some anions are completely immobile while some others are very mobile. The role of the cation sublattice is explained in detail

Cation composition effects on oxide conductivity in the Zr(2)Y(2)O(7)-Y(3)NbO(7) system.

Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the effect of cation composition on the ionic conductivity in the Zr(2)Y(2)O(7)-Y(3)NbO(7) system and to link the dynamical properties to the degree of lattice disorder. Across the composition range, this system retains a disordered fluorite crystal structure and the vacancy concentration is constant. The observed trends of decreasing conductivity and increasing disorder with increasing Nb(5+) content were reproduced in simulations with the cations randomly assigned to positions on the cation sublattice. The trends were traced to the influences of the cation charges and relative sizes and their effect on vacancy ordering by carrying out additional calculations in which, for example, the charges of the cations were equalized. The simulations did not, however, reproduce all of the observed properties, particularly for Y(3)NbO(7). Its conductivity was significantly overestimated and prominent diffuse scattering features observed in small area electron diffraction studies were not always reproduced. Consideration of these deficiencies led to a preliminary attempt to characterize the consequence of partially ordering the cations on their lattice, which significantly affects the propensity for vacancy ordering. The extent and consequences of cation ordering seem to be much less pronounced on the Zr(2)Y(2)O(7) side of the composition range

Thermal conductivity of ionic systems from equilibrium molecular dynamics.

Thermal conductivities of ionic compounds (NaCl, MgO, Mg(2)SiO(4)) are calculated from equilibrium molecular dynamics simulations using the Green-Kubo method. Transferable interaction potentials including many-body polarization effects are employed. Various physical conditions (solid and liquid states, high temperatures, high pressures) relevant to the study of the heat transport in the Earth's mantle are investigated, for which experimental measures are very challenging. By introducing a frequency-dependent thermal conductivity, we show that important coupled thermoelectric effects occur in the energy conduction mechanism in the case of liquid systems