Atom Transport under Varying Pressure

Study of the effect of hydrostatic pressure on the rate of diffusion-limited processes gives in­ formation regarding the intrinsic volume changes associated with formation and motion of defects. While there are basic problems associated with interpretation of the motional volume, such effects may be measured directly in some cases and subtracted to give unambiguous results for the defect formation volume. Experimentally, precise measurement of specimen temperature within a highpressure environment poses the most severe practical constraint. Results are given for measurements to pressures of 7 kilobars of ionic conductivity in a series of pure and doped alkali halides: NaCl, KC1, NaBr, and KBr, all of which are predominantly cationic vacancy conductors. The activation volumes are found to be pressure dependent in the intrinsic regions. In all cases the Schottky pair vacancy formation volume is found to be larger than the molar volume, the ratio being largest for NaCl and smallest for KBr. Results are also given for the total activation volume for diffusion of Ti44 in beta-Ti, a case of “anomalous” bulk diffusion. The tracer penetration plots do not follow a perfect gaussian profile. However, if the penetration plots are fit to a single gaussian, the activation volume derived is consistent with that expected for a vacancy mechansm in bcc metals.