Evidence of hydrogen anti-trapping behaviour by manganese impurities in titanium dihydride from measurements of the proton spin relaxation time

Abstract

The behaviour of manganese in titanium hydride has been studied by NMR to determine the effects which this alloying addition has on hydrogen distribution and diffusion. Measurements of the temperature dependence of the proton spin-spin and spin-lattice relaxation times T1 and T2 were made on two series of TiHx samples, one at fixed hydrogen concentration with varying Mn content and the other at fixed Mn content with varying hydrogen concentration. The measurements were compared with those made on pure TiHx to determine the effects of the Mn. The relaxation rate R1p due to the proton-Mn-moment interaction increases sharply with increasing hydrogen concentration, showing that Mn, which is not a hydride-forming metal, is avoided by the hydrogen. Increasing the hydrogen concentration forces it closer to the Mn and increases the relaxation rate. Thus, the Mn ions act as 'anti-trapping' centres. At intermediate temperatures the slopes of the R1p curves yield an activation energy Ea=0.11 eV/atom, independent of the hydrogen concentration. This value is much less than that appropriate to regions far from Mn ions where Ea=0.50 eV/atom in both the pure and Mn-alloy samples, showing that hydrogen hopping motion in the vicinity of the Mn anti-traps is much faster than in the bulk. These measurements demonstrate clearly the anti-trapping character of Mn as an alloy addition in a hydride-forming host metal and the strong modification of hydrogen hopping motion that it causes.