Selective Thermal Evolution of a Native Oxide Layer in Nb and Nb₃Sn-Coated SRF Grade Nb: An In Situ XPS Study

This contribution discusses the results of an in situ X-ray photoelectron spectroscopy study of the thermal evolution of the native oxide layer on Nb₃Sn and pure Nb. X-ray photoelectron spectroscopy (XPS) data were recorded with conventional spectrometers using an Al Kα X-ray source for spectra collected up to 600 °C and a Mg Kα X-ray source for temperatures above 600 °C. The effect of the thickness, composition, and thermal stability of that oxide layer is relevant to understanding the functional properties of superconducting radiofrequency (SRF) cavities used in particle accelerators. There is consensus that the oxide plays a role in the surface resistance (R_s). The focus of this study is Nb₃Sn, which is a promising material that is used in the manufacturing of SRF cavities, as well as in quantum sensing, and pure Nb, which was included in the study for comparison. The thermal evolution of the oxide layer in these two materials is found to be quite different, which is ascribed to the influence of the Sn atom on the reactivity of the Nb atom in Nb₃Sn films. Nb and Sn atoms in this intermetallic solid have different electronegativity, and the Sn atom can reduce the electron density around neighboring Nb atoms in the solid, thus reducing their reactivity for oxygen. This is shown in the thickness, composition, and thermal stability of the oxide layer formed on Nb₃Sn. The XPS spectra were complemented by grazing incident X-ray diffraction (XRD) patterns collected using European Research Synchrotron facilities. The results discussed herein shed light on oxide evolution in the Nb₃Sn compound and guide its processing for potential applications of the Nb₃Sn-based SRF cavities in accelerators and other superconducting devices.