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Composite Al2O3 and ZrO2 coatings display a wide range of properties depending on the applied deposition methods and on the structural phases of the resulting films. Despite the technological importance of these systems, relatively little work has been performed to understand their properties. One particular intriguing result is the stabilization of cubic ZrO2 in composite Al2O3 and ZrO2 coatings, although pure ZrO2 coatings mainly are monoclinic. The stabilization of the cubic phase has been associated with the following reasons: (i) oxygen understoichiometry (vacancies) and (ii) elastic restraint. One further reason to the stabilization of the cubic phase which we have considered is the solid solution of Al into the monoclinic phase. Motivated by these hypotheses, we have performed first-principle calculations of the energetics of Al atoms substituted at Zr sites and of oxygen vacancies in monoclinic ZrO2. The calculations were performed within the framework of the generalized gradient approximation to density functional theory by using the projected augmented wave (PAW) method, as implemented in the Vienna ab-initio simulation package (VASP). It was found that neither the neutral Al substitutional doping nor the oxygen vacancies are energetically favorable in this system. In fact, the negligible solubility of Al into monoclinic ZrO2 is well known from phase diagram data. Concerning oxygen vacancies, it is possible that as Al substitution takes place in monoclinic ZrO2, the energy of the doped system increases and thus oxygen vacancies may be formed in order to lower this energy. This configuration will also be presented and discussed in our paper. Another important result is that Al does not adopt the 7-fold coordination with oxygen atoms as Zr does; instead it forms a distorted octahedral coordination. Charged defects are likely to be important for the stability of monoclinic ZrO2, since Al has less valence electrons than Zr. To account for the lack of electrons, one extra electron was added to the doped system. We will address the effect of adding extra electrons to the doped system in our paper.
We would like to acknowledge Sandvik Tooling and the Swedish Research Council for financially supporting this research project.
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