Autodiffusion en volume dans la phase plastique de l'acide pivalique (acide 2.2 dimethyl propanoique)—I: Mesure des coefficients de diffusion et de l'effet isotopique

Abstract

Self-diffusion studies have been performed in the orientationally disordered or the so-called plastic phase of pivalic acid. Single crystals of high purity (99.9999%) containing 10 9−10 10 dislocations m −2 have been used. Thin layers of pivalic acid labelled with 14C or tritium were deposited on sample surfaces. Concentrationpenetration curves were established by serial sectioning. Lattice self-diffusion coefficients D, were measured from 281 to 304.75K. At 281K, the value of D is independent of time. From 281 to 301K, D is given by: D(m 2S −1) = (4.9 ± 0.3)10 −4 exp [− (59± 1) kJ mole −1/ RT]. The activation enthalpy of the lattice self-diffusion is roughly equal to the heat of sublimation ( L s = 57 kJ mole −i) and in good agreement with values obtained from NMR. The mass factor f Δ K, where f is the correlation factor and Δ K is a correction factor, has been measured using isotope effect studies. Between 281 and 301K the value obtained is fΔ K* 0.1 +0.2 −0.1