  Superconducting niobium serves as a key enabling material for superconducting
radio frequency (SRF) technology as well as quantum computing devices. At room
temperature, hydrogen commonly occupies tetragonal sites in the Nb lattice as
metal (M)-gas (H) phase. When the temperature is decreased, however, solid
solution of Nb-H starts to be precipitated. In this study, we show the first
identified topographical features associated with nanometer-size hydride phase
(Nb1-xHx) precipitates on metallic superconducting niobium using
cryogenic-atomic force microscopy (AFM). Further, high energy grazing incidence
X-ray diffraction reveals information regarding the structure and stoichiometry
that these precipitates exhibit. Finally, through time-of-flight secondary ion
mass spectroscopy (ToF-SIMS), we are able to locate atomic hydrogen sources
near the top surface. This systematic study further explains localized
degradation of RF superconductivity by the proximity effect due to hydrogen
clusters.
