The serial sectioning technique was used to study the effect of hydrostatic pressures of 6.2kbar upon the rate of 22Na diffusion in high-purity single crystals at 663 to 731C (figure 27). The cation diffusion process was characterized by a pressure-dependent activation volume which varied from about 37cm3/mol at low pressures to about 25cm3/mol at high pressures. When possible divacancy contributions were neglected, this implied a formation volume for Schottky defects of about 2.2 molar volumes.

Pressure Dependence of Self-Diffusion of 22Na in NaCl. G.Martin, D.Lazarus, J.L.Mitchell: Physical Review B, 1973, 8[4], 1726-31