Extended in situ Cu-K XAFS and XRD study on the site preference and valence of copper ions in (Mg_(1-x)Cu_x)O

摘要

The local environment and valence of copper ions in copper-doped magnesium oxide, (Mg_(1-x)Cu_x)O, were investigated under in situ conditions as a function of temperature T, composition x, and oxygen activity a_(O(2)). By comparison of Cu-K extended X-ray absorption fine structure (EXAFS) data with corresponding multiple scattering EXAFS simulations, copper was proven to occupy preferentially magnesium lattice sites. The bivalency and the site preference of copper was inferred from a comparative X-ray absorption near edge structure (XANES) study using standards containing copper in various oxidation states and coordination geometries. Strong similarities between the Fe-K XANES of Fe~(2+) in (Mg_(1-x)Fe_x)O and the discussed Cu-K XANES support these interpretations. Additionally, the Jahn-Teller distortion was estimated to be small at room temperature. As the site preference and valence of copper is not influenced by variation of the experimental conditions, the local thermal expansion around copper in (Mg_(0.948)Cu_(0.052))O was characterized by quantitative EXAFS analysis. The thermal expansions extracted from the nearest and next-nearest radial distribution function coincide with those determined from in situ recorded X-ray diffraction data. Furthermore, the Cu-K step height Δμd is found to correlate with the lattice parameter a of (Mg_(0.948)Cu_(0.052))O, indicating the step height change to be also a measure of thermal expansion.