Metallic thallium vs thallium iodide vapour pressure

[dor123]

Metallic thallium have so low vapour pressure, that thallium spectral lamps like that of Osram, have their arctubes enclosed in a metal cylinder for heat conversion: http://www.lamptech.co.uk/Spec%20Sheets ... l%20Tl.htm
In comparison: Thallium iodide have so high vapour pressure that green MH lamps usually have no heat reflective coatings at the end of their arctubes, can run up and hot restrike very fast compared to white MH lamps of the same wattage: http://80.229.24.59:9232/gallery/displa ... ?pid=20790
How this is the happens?