As an alternative, what's the heaviest element they could use that is safe for the environment?
Iodine has almost the same atomic mass as xenon, but it is much easier to store than xenon (it is solid, but much less dense than mercury), even if it is corrosive.
The elements with a closer atomic mass to mercury are difficult to vaporize. It would not be impossible to use any of them in an ionic thruster, but it would be extremely difficult to obtain a high enough ionic flux and a long enough lifetime of the engine.
The question was which are the heaviest usable elements, and those are iodine and xenon.
Iodine is also cheap. However the fact that it is not an inert gas like krypton and xenon creates some difficulties, even if those are balanced by its easier storage.