I have synthesized a Zeolitic Imidazolate Framework crystal.
When analyzing it with a combination of DSC and X-RD (quenching and analyzing at different temperatures), I could see it become amorphous prior to melting, because all bragg peaks disappeared. How is this possible?
The litterature suggests that only crystalline regions can melt, whereas amorphous regions will show a glass transition.
I think that first, you have to see if this process reversible and the sample is not decomposing (it might be that you see the decomposition by DSC) or dehydration (if there is coordination or hydration water) or an effect like that.
Another think that you must take into account is that the structure can lose the long range order (and therefore the Bragg peaks disappear) but the short range order (nanocrystallinity) can remain and be the cause of the melting peak.
I hope that this can help.
Did you check the DSC thermogram at the temperature you see the amorphous?
There has to be and edothermis peak. It doesn´t make sense to see it before the melting point. Is the DSc well calibrated?. Are you sure that both happen at the same temperature?
The DSC should be calibrated correctly.
The thermogram has an endotherm peak, followed at a later point by an exothermic peak, followed by melting and decomposition. I assume that the first endothermic peak is either due to thermal amorphization followed by some degree of recrystallization, which would cause the melting peak. Otherwise, as Fransisco kindly suggested, maybe the nanocrystallinity is retained, causing the melting. I think perhaps the latter is more probable, as the heating / cooling rates are too fast for the recrystallization
I am not very familiar with DSC, but there are studies about amorphization of ZIFs. Could these publications help in unraveling the issue?
http://www.nature.com/ncomms/2015/150828/ncomms9079/full/ncomms9079.html
http://onlinelibrary.wiley.com/doi/10.1002/anie.201007303/full