Alexandre Malinge

Have you done the experiment with the pan only (no sample)?

What are the pans made of and what is your carrier gas?

I add a reference pan, the gas was nitrogen and the pans are in Al.

May be you should try to calibration of the instrument and try to repeat it after or may be a comparative study I'm areal condition and inert nitrogen condition help you also you may check the gas or you sample if it contains moisture...

I also suggest you to take your sample to an oven, under N2 atmosphere, at 300 ºC and take some infrared spectra (compare using Al pan and a ceramic crucible). Aluminiun likes a lot halogens, the infrared can help you solving this puzzle.

Dear Alex, are you sure its decomposing?

1)Your axis show exo down?

2) If your sample decompose you will normally see a shift in the baseline because you physically loose mass.

3) Open the pan after cycle 1 and after the second cycle. Observe

4) Can you run this on a TGA?

Alexandre Malinge

That wasn't my question. Repeat the experiment with your reference pan but no sample in your sample pan. Check the system carefully for leaks.

The behavior you see is actually quite common for glass-forming compounds (many "small molecules" qualify).

The first compound, initially crystalline, does not recrystallize after cooling from the melt. Maybe it would if given plenty of time or very slow cooling. In any case, in the second cycle you are dealing with a stable glass that doesn't devitrify on heating, so the melting transition is gone.

The second compound, also initially crystalline, also "freezes" into a glass when cooled from the melt. In this case however, the glass is somewhat less stable and it devitrifies on reheating between the glass transition and the melting point. Therefore the melting transition reappears in the second cycle. Another interesting fact - I have seen this before - is that the long-annealed crystalline compound displays a solid-solid phase transition below the melting point, but that transition is lost, or "merged" with the melting point, in the second cycle, and the new melting point is a bit lower than the old one. This is probably due to the crystalline material derived from devitrification being more disordered than the initial crystalline form. You can verify this with XRD.

t.I. Generally impure liquids form more stable glasses than pure ones. You can try a different nobler metal (gold or platinum) and see. is also possible that a bit of decomposition on melting stabilizes the glass. But it is also possible that the tendency to form a glass on cooling from melt is reinforced by interaction with the aluminum pan, as suggested by Luiz Fernando de Sousa Lima

Thanks for all your interesting answers. It's very usefull to understand what happened in these compounds.

Good afternoon,

please, write all of your questions on my e-mail:

[email protected]

Best wishes,

Irina