None of these are really the primary reason why we use noble gasses. It has to do with the ionization energy of the gas, which tells us how much energy we have to spend per molecule to ionize the gas.
http://aerospace.mtu.edu/__reports/Conference_Proceedings/20...
EDIT: and iodine, ie the subject of this study, is also significantly better than Xenon in this metric: “ A possible alternative is iodine, which is much more abundant and cheaper than xenon and can be stored unpressurized as a solid. In addition, both atomic and diatomic iodine have a lower ionization threshold, and diatomic iodine has a relative mass that is almost twice that of xenon.”
So I’m imagining a solid block of iodine (it is in cold space), and when the engine needs thrust, some current from the solar cells is passed through a heating element to drive sublimation + lots of complex rocket science that I do not know of.
If you are referring to the noble gases, I don’t think they are cooled to the point of solids; just high pressure liquids..
You can technically liquefy/boil iodine (I've done it a few times). It happens around 110C IIRC.