Mass dependence of diffusion in a supercooled metallic melt

The isotope effect E = (D-alpha/D-beta - 1)/(root m(beta)/m(alpha) - 1) of cobalt diffusion in the deeply supercooled melt of the metallic alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 has been measured employing the radiotracers Co-57 and Co-60. The isotope effect is very small, E = 0.09 +/- 0.03, and exhibits no significant temperature dependence in a range up to 120 K above the calorimetric glass transition temperature T-g, encompassing almost 3 orders of magnitude in the diffusivity. This result suggests that long-range diffusion in the deeply supercooled melt is not mediated by viscous flow but rather proceeds by collective hopping processes involving about ten atoms.