Résumé du preprint DAPNIA-04-341

In-situ investigation of the Nb(110)/oxygen interface
M. Delheusy, A. Stierle, C. Antoine, N. Kasper, I. Costina, H. Dosch
Interstitial oxygen impurities are known to affect strongly the physical properties of niobium and in particular its superconducting properties . The investigation of the oxygen defect distribution and the structure which is induced in the vicinity of the metal/oxide interface under different oxidation conditions is therefore crucial for understanding the near surface region properties of the material. This is of first importance for numerous superconducting high technology applications and  especially for the future improvements of high frequency (HF) superconducting cavities made of niobium. The HF cavities performances are indeed directly dependant on the last surface treatments applied on them (polishing, thermal treatments)  and most of their limitations occur in a small region of the surface where  the HF field penetrates (~50-100nm). It has been shown previously that single oxygen impurities randomly distributed in niobium bulk are nuclei for the formation of a trigonal phase, the  phase. The occurrence of local -phase-domains around the oxygen defects is intimately related to the common instability of the bcc lattices for lattice strain in its [111] direction and gives rise to a specific diffuse scattering , . X-ray surface sensitive techniques allow to perform in-situ depth-resolved measurements in the near surface region, in particular of this diffuse scattering . In this work we investigate the oxygen impurities distribution and the structure for the Nb(110)/oxygen interface for different thermal treatments in ultra high vacuum, prior to and after an oxidation under 5.10-6mbar of dry oxygen. Diffuse Scattering and Crystal Truncation Rods (CTR) measurements have been performed under Grazing Incidence X-ray Diffraction (GIXD) geometry, as well as X-ray Reflectivity measurements. On the poster, we will present how the interfacial structure, its roughness, the oxygen depth profile and the distortions induced in the Nb lattice close to the interface were monitored using these in-situ X-ray Diffraction techniques at the MPI surface diffraction beamline at the synchrotron source ANKA. 
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