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Pulkit Garg1 Shreyas Balachandran2 Ilaksh Adlakha4 1 Peter Lee2 Thomas Bieler3 Kiran Solanki1

1, Arizona State University, Tempe, Arizona, United States
2, Florida State University, Tallahassee, Florida, United States
4, Indian Institute of Technology Madras, Chennai, , India
3, Michigan State University, East Lansing, Michigan, United States

The absorption of hydrogen and subsequent precipitation of hydrides degrades the superconductive properties of niobium. Although addition of dopant elements, particularly nitrogen, have shown improvement in the quality factor of superconducting radio frequency (SRF) niobium cavities, the underlying mechanisms associated with kinetics of hydrogen and the thermodynamic stability of hydride precipitates are not well known. Here, through first principles calculations, we report the effects of nitrogen doping on the energetic preference for hydrogen to occupy interstitial sites and the stability of niobium hydride. In particular, we show that the presence of nearby nitrogen significantly increased the energy barrier for hydrogen diffusion from one particular tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable up on addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, we find that nitrogen has a tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to an unstable state for both hydrogen and hydride. This explains the experimental observation i.e., niobium hydride is less stable in nitrogen doped niobium. Therefore, the presence of nitrogen during the processing of SRF cavities can play a critical role in controlling hydride precipitation and subsequent SRF properties.

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