Iodine? Keep it cold and it stays solid. But it doesn’t liquify; it is one of the elements that can sublimate from solid to gas (not sure of the physical chem here, but the transition temp to liquid vs gas from solid is so close that it doesn’t have a distinct liquid phase(?)).

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..