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 Al[sub 2]O[sub 3] substrate of an Al[sub 2]O[sub 3]/Cr[sub 2]O[sub 3](001) interface has fine structure when the x-ray energy is tuned through the Cr [ital 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 Cr[sup 3+] oxidation state. We demonstrate how the near-edge energy dependence of the CTR diffracted intensity may be used to deduce the amplitude [ital and] [ital 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 Cr[sub 2]O[sub 3]/Al[sub 2]O[sub 3] interface. While the local atomic coordination at the interface is the same as that of Al[sub 2]O[sub 3] and Cr[sub 2]O[sub 3], there is a stacking fault at the interface covering half of the interface area.