Direct atomic-scale imaging of hydrogen and oxygen interstitials in pure niobium using atom-probe tomography and aberration-corrected scanning transmission electron microscopy.

Imaging the three-dimensional atomic-scale structure of complex interfaces has been the goal of many recent studies, due to its importance to technologically relevant areas. Combining atom-probe tomography and aberration-corrected scanning transmission electron microscopy (STEM), we present an atomic-scale study of ultrathin (~5 nm) native oxide layers on niobium (Nb) and the formation of ordered niobium hydride phases near the oxide/Nb interface. Nb, an elemental type-II superconductor with the highest critical temperature (T(c) = 9.2 K), is the preferred material for superconducting radio frequency (SRF) cavities in next-generation particle accelerators. Nb exhibits high solubilities for oxygen and hydrogen, especially within the RF-field penetration depth, which is believed to result in SRF quality factor losses. STEM imaging and electron energy-loss spectroscopy followed by ultraviolet laser-assisted local-electrode atom-probe tomography on the same needle-like sample reveals the NbO(2), Nb(2)O(5), NbO, Nb stacking sequence; annular bright-field imaging is used to visualize directly hydrogen atoms in bulk β-NbH.