Oxidation state of a buried interface: Near-edge x-ray fine structure of a crystal truncation rod

We observe that the crystal-truncation-rod (CTR) diffraction from the Al₂{O}₃$ substrate of an ${Al}₂O₃/Cr₂{O}₃$(001) interface has fine structure when the x-ray energy is tuned through the Cr K absorption edge. This fine structure is attributed to interference between scattering from the substrate and Cr atoms at the interface, which are coherent with the substrate. Comparisons with x-ray-absorption spectra of standard samples demonstrate that the Cr at the interface is predominantly in the Cr³⁺ oxidation state. We demonstrate how the near-edge energy dependence of the CTR diffracted intensity may be used to deduce the amplitude and phase of the interface scattering. This phase information is used in conjunction with reciprocal-lattice-vector-dependent measurements of diffracted power due to CTR's to deduce the structure of the Cr₂{O}₃$/${Al}₂O₃ interface. While the local atomic coordination at the interface is the same as that of Al₂{O}₃$ and ${Cr}₂O₃, there is a stacking fault at the interface covering half of the interface area.