Parallel-sheets model analysis of space charge layer formation at metal/ionic conductor interfaces

The space charge formation at metal/yttria-stabilized zirconia interface is investigated by combining defect formation energies from first principles with a parallel-sheets model. In this model, the crystal orientation and interlayer distances between crystal planes are taken into account explicitly. This is in contrast to previous space charge models, which utilized a continuum approximation. The crystal-plane-resolved picture of the space charge is discussed in detail. It is found that the results from the parallel-sheets model resemble closely those of the continuum model. We find this to be the case even for the polar (111) orientation of cubic zirconia, despite the fact that continuum approximations implicitly assume a homogeneous system. We also examine the effect of yttrium dopant segregation on the space charge utilizing the parallel-sheets model. It is found that dopant segregation tends to decrease the width of the oxygen vacancy depletion layer that occurs in oxidizing atmosphere, and that it tends to increase the accumulation of vacancies in reducing atmosphere.