We have investigated the dissolution of a natural oxide layer on a Nb(110) surface upon heating, combining x-ray reflectivity, grazing incidence diffuse scattering, and core-level spectroscopy. The natural oxide reduces after heating to 145°C partially from Nb2O5 to NbO2, and an enrichment in subsurface interstitial oxygen by ∼70% in a depth of 100Å is observed. After heating to 300°C, the oxide reduces to NbO and the surplus subsurface oxygen gets dissolved into the bulk. Our approach can be applied for further investigation of the effect of subsurface interstitial oxygen on the performance of niobium rf cavities.

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