Local hydrogen vibrations in Nb in the presence of interstitial (N,O) and substitutional (V) impurities

We have measured the hydrogen vibrations in NbV0.008{H}0.005$, Nb${N}0.004H0.003, and NbO0.011{H}0.010$ between 295 and 4 K. In the case of Nb${V}0.008H0.005, these spectroscopic measurements suggest a down shift of the phase boundary for H precipitation by about 30 K. At low temperatures we find excitations at 117 and 166 meV, which are characteristic of an ordered hydride phase. On the contrary, O or N interstitial defects totally suppress the precipitation of H due to trapping. The vibrational energies of H trapped by N (or O) are 106 (107) and 160 (163) meV, which are very close to energies measured for free H in NbH0.005 at 295 K. This clearly indicates that H atoms in these systems are trapped at relatively unperturbed tetrahedral-type sites. The peak shapes are distinctly different for the trapping and the nontrapping defects. Several mechanisms that can cause the large widths observed are discussed.