A robust oxide terminates Nb surfaces and plays a crucial role in the fabrication of superconducting radio frequency (SRF) cavities for particle accelerators and light sources. The RF fields only penetrate ~100 nm through the cavity surface, thus SRF cavity performance is influenced by the surface chemistry and quality. High resolution helium atom diffraction and Auger spectroscopy measurements on a model surface oxide system, Nb(100)-(3x1)-O, show that the atomic-scale surface structure, composition, and coherence of the oxide overlayer remain unchanged up to at least 1130 K. These results indicate that the oxide plays a significant role in the interfacial properties of Nb at elevated temperatures, and the oxide must be taken into account during doping and the chemical mechanisms that govern the growth of high-quality materials, including alloying for SRF cavities. Helium atom scattering and density-functional theory are used to characterize the phonon band structure of the (3x1)-O surface reconstruction of Nb(100), revealing the surfaces interatomic forces and bonding. Innovative DFT calculations comparing surface phonons of bare Nb(100) to those of the oxide surface show increased resonances for the oxide and generate calculated dispersion curves that align well with experimental results and yield atomic displacements. Inelastic helium time-of-flight measurements show phonons with mixed longitudinal and shear-vertical displacements along both the &lt;100&gt;, and &lt;110&gt;, symmetry axes over the entire first surface Brillouin zone. Force constants calculated for bulk Nb, Nb(100), and the (3 x1)-O Nb(100) reconstruction indicate much stronger forces and bonding in the oxide surface and reveal interatomic interactions that cause the (3x1)-O to passivate and stabilize the surface, inhibiting surface melting and alloying. These results represent, to our knowledge, the first high temperature atomic scale structural measurements of the oxide surface, the first phonon dispersion data for the oxide surface, and the first ab initio calculation of the oxide’s surface phonons. These studies supply critical information for the further development of advanced materials for SRF cavities.

High temperature persistence and bonding of the Nb(100) surface oxide reconstruction shown by high resolution helium diffraction, inelastic time-of-flight measurements, and density functional theory calculations

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ACS Fall 2022

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