Hydrogen vibrations and hydrogen trapping in TaN0.006H0.003

We studied, by neutron spectroscopy, local vibrations of hydrogen interstit ials and hydrogen trapping by nitrogen interstitials in TaN0.006H0.003 and in a nitrogen-free TaH0.086 reference sample between 1.4 and 295 K. The mea surements on TaH0.086 yielded vibrational energies E-1 = (113 +/- 1) meV an d (doubly degenerate) E2,3 similar or equal to 163 meV at 295 K where hydro gen is in solid solution, together with a higher energy E-1 = (120 +/- 1) m eV and essentially unchanged energies E2,3 at low temperatures (1.4 to 4.2 K) where hydrogen precipitates in a hydride phase. The energies agree with previous studies. The measurements on TaN0.006H0.003 yielded, at 295 K and at low temperatures, vibrational energies identical to those of the nitroge n-free sample at 295 K. The results show that the hydrogen interstitials ar e trapped by the nitrogen at low temperatures and do not precipitate. They show further that the trapped hydrogen interstitials occupy tetrahedral sit es, i.e, the same type of site as occupied in the absence of traps.