XPS analysis of the surface composition of niobium for superconducting RF cavities

The surface chemical composition of Nb samples prepared by following procedures commonly employed in the preparation of RF cavities has been studied by XPS and ARXPS. In order to understand the process occurring at surface, the thermal evolution of the surface has been studied in the 30°C÷1000°C range of temperature both in the heating and cooling phases. During the heating phase, an irreversible transition has been observed near T≅200÷300°C; it is characterized by a progressive reduction of Nb oxides from Nb2O5 to NbO2 and finally to NbO. The O signal disappears near T≅1000°C and it reappears below 900°C, during the cooling phase. This can be interpreted as due to O migration in the Nb matrix, raising T, and to O diffusion towards the surface as the temperature decreases. Work function measurements show that the values of Φ are strongly correlated with the chemical composition of the surface.