Examen de la loi d'arrhenius de la diffusion dans les solides

Validity of the Arrhenius law has been investigated by measuring the influence of temperature on the transport processes in very pure single crystals of sodium iodide. For the first time, due to the high experimental accuracy and the precision of the temperature control, diffusion coefficients could be measured in a solid in steps of a few degrees up to a few tenths of a degree of the temperature of fusion. We have studied: (a) ionic conductivity, (b) self-diffusion of the cation Na⁺ and (c) self-diffusion of the anion I^?. The three transport processes follow the same temperature dependence: (a) strict verification of the Arrhenius law over several orders of magnitude up to about thirty degrees below the melting point and (b) in the last thirty degrees a positive departure from the simple exponential law is observed which increases rapidly to reach 50% at fusion. The different possible reasons of this departure are discussed. The phenomenon seems to be connected with the lattice dynamics before melting.