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.