Effect of hydrostatic pressure on self-diffusion and plastic deformation in plastic crystals

Abstract

A study has been made of the influence of hydrostatic pressures in the range 1–60 MN m^–2 on the processes of radiotracer self-diffusion in hexamethylethane and pivalic acid and high temperature plastic deformation (creep) in cyclohexane, pivalic acid, perfluorocyclohexane, tetra(fluoromethyl)-methane, camphene, succinonitrile, hexamethylethane and phosphorus. These rotator-phase solids have entropies of fusion in the range R-2.5R. The close similarity of the activation volumes, V⁺_d, derived from the pressure dependence of both processes in hexamethylethane and pivalic acid confirms that high temperature plastic deformation is self-diffusion controlled. The values of V⁺_d lie in the range 1–1.9 Ω(where Ω is the molar volume of the solid). There are distinctions between face centred cubic and body centred cubic solids but in general the increase parallels the increasing entropy of fusion of the solids. The variation probably reflects the contribution of varying proportions of vacancy and divacancy diffusion and possibly diffusion along grain boundaries and dislocations to the total observed diffusion coefficient.