Also, when we watch Stallone get frozen, it is clearly via chemical reaction that takes whatever Zima or whatever is pumped into his tank and turns it solid with the addition of the little blue thing. So it's a chemical process, not a thermodynamic process. In other words, they're not putting him in a literal freezer, they're placing him in suspended animation. Nonetheless, 36 years later, the place is so cold, and apparently so poorly designed to deal with the stresses of it's own environment, that everything in the prison is covered in frost. Also, the liquid that covers Phoenix at the end is the same liquid that is used to put people in suspended animation. So, again, it is designed to KEEP PEOPLE ALIVE, but when it is activated this time it suddenly behaves like liquid nitrogen, and Phoenix shatters like the liquid metal Terminator.