Activation Volume ofC57oDiffusion in AmorphousCo81Zr19

We have measured the pressure and temperature dependence of ${}^{57}\mathrm{Co}$ diffusion in structurally relaxed ${\mathrm{Co}}_{81}{\mathrm{Zr}}_{19}$ glass. The activation enthalpy and the preexponential factor are 1.4 eV and $8.4\times {10}^{-9\mathrm{}}{\mathrm{m}}^2{\mathrm{s}}^{-1\mathrm{}}$. The pressure dependence is extremely small and yields an activation volume of $V_{\mathrm{act}}=(0.08\pm 0,1)\Omega$, where $\Omega$ is the mean atomic volume. These results allow us to rule out that diffusion at this alloy composition is mediated by vacancylike thermal defects, as recently suggested for Zr-rich metal-metal glasses [A. Grandjean et al., Phys. Rev. Lett. 78, 697 (1997)]. The validity of the Keyes relationship is critically discussed.