I performed a TG-DTA measurements of cerium oxide prepared by precipitation from cerium nitrate and then calcinated at 450°C. The sample don't have significant weigt losses, but have a DTA exothermic peak at about 230°C, it is possible due to crystalizzation of amorphous portion? The measurements were made in nitrogen stream from 20°C to 600°C with a ramp of 20°C/min.
A paper by Dos Santos et al. (attached) supports your explanation ("An exothermic peak around 270–350 °C, correlated to a weight loss, that must be considered as due to the crystallization of the residual amorphous phase.").
In my opinion, the reasoning is a bit strange (weight loss + crystallization), and there are no additional data supporting this conclusion. Nonetheless, assignment of the peak to the crystallization of amorphous portion sounds plausible; another possible explanation can be a recrystallization of the sample. A DTA/TG record only rarely provides you with sufficient information.
First thing to check is if the TG-DTA is contaminated. If you run a blank and still see the peak at 230 ° C, then the system is contaminated. If there is no change in the system, then it is likely to be from your sample.
I have to ask this because TGA while being a powerful tool, can also be prone to false information when previous users don't clean up the system properly.
One way to determine if it is indeed due to crystallization, is to perform an XRD on the starting material and final material.
Also is the TG-DTA coupled to a MS detector? It would be interesting to see what gases are evolved from the starting material during heating.
I thank everyone for their helpful advice, I will do other measurements to make sure to the assignment of the signal.
Best Regards
Hi...
Because no mass loss has been associated to this exothermic peak, it should be considered as a phase transformation, and so it is probably that not well crystallized residual cerium oxide species (from your initial synthesis) can follow the changes toward a more stable structure (CeO2).
I recommend you this paper:
H. H. Ko et al. (2014). Thermal behavior and crystallization kinetics of cerium dioxide precursor powders. Ceramics International, 40, 13953–13959.
Kind regards
Heavy Lanthanide elements (like Cerium) when in an oxide form can behave unpredictably when exposed to changes in environment (change in temperature in DTA). It is possible that the peak at 230 degrees was due to a structural rearrangement at the surface of the crystal which resulted in an energy penalty as the crystal relaxed. This would account for a change in energy without there being a reduction in mass.
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