Diffusion of mercury and cadmium in lead at high pressures

The diffusion of Hg and Cd in solid lead has been measured at hydrostatic pressures to 40 kbar for temperatures between 250 and 550°C using the radiotracer sectioning technique. The data were analyzed assuming a single effective mechanism of diffusion with a temperature- and pressure-dependent activation volume. The activation energy and activation volume at zero pressure and 600 K are, respectively, $\Delta H=23.10\mathrm{}\pm 0.14$ kcal/mole, $\frac{\Delta V}{V_0}=0.516\mathrm{}\pm 0.006$ for Hg in Pb: $\Delta H=22.17\mathrm{}\pm 0.18$ kcal/mole, $\frac{\Delta V}{V_0}=0.317\mathrm{}\pm 0.010$ for Cd in Pb. The pressure and temperature coefficients of the activation volume along with the specific heat of activation, although quite uncertain are also determined. Even though the activation energies for the diffusion of both Hg and Cd in Pb is near that for lead self-diffusion, the activation volume is too small in both cases to be consistent with a vacancy mechanism.