Hydrogen concentration dependence of tritium tracer diffusion coefficient in alpha phase of niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients D_tof tritium in alpha phase of hydrogenated and deuterized niobium (α-NbH_xT_yand α-NbD_xT_y(x<0,8, y<<x)) have been measured at 473 K, 493 K and 553 K. The data on D_tshow typical hydrogen concentration dependence: D_tof tritium for both α-NbH_xT_y, and α-NbD_xT_y, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of D_tof tritium differs from that of D_tof protium in α-NbH_xor of deuterium in α-NbH_xT_y. On the other hand, no appreciable differences in the concentration dependence of D_tof tritium between α-NbH_xT_y, and α-NbD_xT_y, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that D_tof tritium for a tritiated niobium (α-NbT_x) is not very different from that for α-NbH_xT_yand α-NbD_xT_y. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained D_tof tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.