Modeling the solid state reaction CO2·C2H2 → CO2+C2H2

Crystalline CO2·C2H2 is metastable with respect to solid CO2 and solid C2H2. Infrared spectra of the bending regions of the three solids are presented and it is shown that the solid phase decomposition of CO2·C2H2 preserves the integrity of the cryofilm sample. Although the plot of fractional decomposition against time is sigmoidal it is not well described by the Avrami equation. It is shown that the kinetics of the transformation are well described by a growth in three dimensions, unimpeded until reactant material is locally exhausted.