Uranium?
Nitroglycerin is a material known for its rapid decomposition, releasing energy in the form of an explosion virtually instantaneously.
explosive
Nitroglycerin is a material capable of rapid decomposition, releasing a vast amount of energy virtually instantaneously. This explosive compound is highly unstable and is commonly used in dynamite and other explosive devices.
One example of a material that can decompose rapidly and release energy instantaneously is trinitrotoluene (TNT). When detonated, TNT undergoes a rapid chemical decomposition that releases a large amount of energy in the form of heat, pressure, and a shockwave.
Explosive
explosive
how rapid of decomposition are we talking? what kind of decomposition? what is causing it? the answer to your question is "any", because there are too many variables left open and therefore infiinite possibilities.
decomposition
Aquatint
A biodegradable is a material which is capable of being decomposed by biological activity.
This might be the description for thermal decomposition.
Probable you think to decomposition.