Activation volume of 57Co diffusion in amorphous Co81Zr19

We have measured the pressure and temperature dependence of Co-57 diffusion in structurally relaxed Co81Zr19 glass. The activation enthalpy and the preexponential factor are 1.4 eV and 8.4 x 10(-9) m(2) s(-1). The pressure dependence is extremely small and yields an activation volume of V-act = (0.08 +/- 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.