  A simple model to describe Mixed Ionic Electronic Conductors (MIEC) in terms
of standard semiconductor physics is described. This model allows to understand
defect equilibrium and charge transport at ideal heterojunctions between
materials simultaneously conducting electronic and ionic point defects and to
explore how rectifying effects on the electronic or ionic currents may affect
the chemical diffusion and voltage at the interfaces under polarization. We
found qualitatively good agreement with experimental measurements of the
electrical conductivity relaxation of La2NiO4+d thin films epitaxially grown on
NdGaO3 (110) substrates when the possible oxygen exchange between film and
substrate is taken into account. We discuss the implications of this model to
understand space charge layer formation and chemical diffusion on oxide thin
film heterostructures when exposed to high temperatures and different oxygen
partial pressures.
