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

Abstract

We have measured the hydrogen vibrations in Nb${\mathrm{V}}_{0.008}$ ${\mathrm{H}}_{0.005}$, Nb${\mathrm{N}}_{0.004}$ ${\mathrm{H}}_{0.003}$, and Nb${\mathrm{O}}_{0.011}$ ${\mathrm{H}}_{0.010}$ between 295 and 4 K. In the case of Nb${\mathrm{V}}_{0.008}$ ${\mathrm{H}}_{0.005}$, 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 Nb${\mathrm{H}}_{0.005}$ 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.