Nuclear-magnetic-resonance studies of hydrogen antitrapping effects by impurities (Mn,Cr,Fe) in zirconium dihydride.

Abstract

We report nuclear-magnetic-resonance measurements of the proton spin-lattice relaxation time ${\mathit{T}}_{1}$ in both pure and paramagnetic impurity-containing zirconium dihydrides. Measurements of the temperature dependence of the proton spin-lattice relaxation times ${\mathit{T}}_{1}$ were made on two series of ${\mathrm{ZrH}}_{\mathit{x}}$ samples (pure and impurity doped). The results show that paramagnetic impurities (Mn,Cr,Fe) contribute an additional spin-lattice relaxation rate ${\mathit{R}}_{1\mathit{p}}$, which is observed to increase in the sequence Fe\ensuremath{\rightarrow}Cr\ensuremath{\rightarrow}Mn and also to increase sharply with increasing hydrogen concentration at fixed impurity content in each case. The former result is consistent with the experimental results that the tendency towards localized moment formation in zirconium increases in the same sequence, whereas the latter may be accounted for by the antitrapping character of these impurities. Furthermore, the fit of ${\mathit{R}}_{1\mathit{p}}$ data for the ${\mathrm{ZrH}}_{1.97}$ sample doped with 500 ppm Mn or Cr yields an activation energy ${\mathit{E}}_{\mathit{a}}$=0.78 eV/atom, which is \ensuremath{\sim}75% of ${\mathit{E}}_{\mathit{a}}$ in the bulk.