  Niobium provides the basis for all superconducting radio frequency (SRF)
cavities in use, however, hydrogen is readily absorbed by niobium during cavity
fabrication and subsequent niobium hydride precipitation when cooled to
cryogenic temperatures degrades its superconducting properties. In the last few
years the addition of dopant elements such as nitrogen has been experimentally
shown to significantly improve the quality factor of niobium SRF cavities. One
of the contributors to Q degradation can be presence of hydrides, however, the
underlying mechanisms associated with the kinetics of hydrogen and the
thermodynamic stability of hydride precipitates in the presence of dopants are
not well known. Using first principles calculations, the effects of nitrogen on
the energetic preference for hydrogen to occupy interstitial sites and hydride
stability are examined. In particular, the presence of nitrogen significantly
increased the energy barrier for hydrogen diffusion from one tetrahedral site
to another interstitial site. Furthermore, the beta niobium hydride precipitate
became energetically unstable upon addition of nitrogen in the niobium matrix.
Through electronic density of states and valence charge transfer calculations,
nitrogen showed a strong tendency to accumulate charge around itself, thereby
decreasing the strength of covalent bonds between niobium and hydrogen atoms
leading to a very unstable state for hydrogen and hydrides. These calculations
show that the presence of nitrogen during processing plays a critical role in
controlling hydride precipitation and subsequent SRF properties.
