Atomic scale mechanism for the Ge-induced stabilization of the tetragonal, very high k, phase of ZrO2

Using x-ray absorption fine structure aided by ab initio structural simulations we demonstrate the atomic scale mechanism responsible for the stabilization of the otherwise unstable and very high-j tetragonal phase of ZrO2 by the incorporation of Ge atoms. In tetragonal ZrO2 the cation has a split first coordination shell formed by eight oxygen atoms. We provide a direct experimental proof that when Ge is incorporated in the oxide, four of the eight O atoms collapse towards Ge giving rise to a local structure strongly reminiscent of that found in quartz-like GeO2, thus stabilizing the tetragonal phase.