Combined In Situ EDXRD/EXAFS Investigation of the Crystal Growth of [Co(C₆H₁₈N₄)][Sb₂S₄] under Solvothermal Conditions: Two Different Reaction Pathways Leading to the Same Product

Abstract

The crystal growth and kinetics of the formation of [Co(C6H18N4)][Sb2S4] were investigated under solvothermal conditions with combined in situ EDXRD (energy dispersive X-ray diffraction) and in situ EXAFS (extended X-ray absorption fine structure) at different temperatures. In the overwhelming number of reactions all product reflections occur simultaneously in the EDXRD spectra. A detailed analysis of the extent of reaction α vs time clearly shows that the mechanism changes during the reaction. Such a change occurs at different temperatures after different reaction times. At the early stages the reactions are controlled by diffusion, and later the reaction exponents m suggest a more complex behavior. At the end of the formation of [Co(C6H18N4)][Sb2S4] a second phase is formed which does not contain Co. The results of the in situ experiments suggest that [Co(C6H18N4)][Sb2S4] is not only a metastable phase but rather that the second product crystallizes because the Co2+ concentration in solution is too low for further formation of [Co(C6H18N4)][Sb2S4]. In the minor cases of the solvothermal reaction a very different growth was observed. At the beginning only one intense reflection occurs in the spectra and after a distinct time all other reflections start to grow simultaneously. This strange behavior indicates that disordered layers are formed at the beginning which start to arrange at later stages, yielding the three-dimensional long-range order. The in situ EXAFS experiments performed at the Sb K-edge demonstrate that, independent of the reaction time, the Sb/S ratio, and the amine applied, only the two species SbS3 and SbS4 are present in solution.