Concerted Diffusion of Molecular Clusters in a Molecular Sieve

We show that attractive interactions between molecules adsorbed in molecular sieves play a vital role in the mechanisms and rates of intracrystalline diffusion. Over a wide temperature range, the single-file diffusion of ${\mathrm{CF}}_4$, ${\mathrm{SF}}_6$, and ${\mathrm{CCl}}_4$ in ${\mathrm{AlPO}}_4$- $5$ is dominated by the diffusion of molecular clusters. ${\mathrm{SF}}_6$ and ${\mathrm{CCl}}_4$ clusters diffuse by concerted mechanisms involving all of the cluster's molecules simultaneously. The activation energies, $E_a$, for these mechanisms are strongly size dependent and are lower than $E_a$ for monomer diffusion.