Antimony-125 decays with a half life of over two years to  which itself decays with a half life of almost two months via isomeric transition to the ground state. While its relatively short half life and the significant gamma emissions (144.77 keV) of its daughter nuclide make usage in an RTG less attractive, Sb-125 could deliver a relatively high power density of 3.4 W/g.
Fluoride volatility can recover antimony as the mildly volatile (solid at room temperature) Antimony trifluoride or the more volatile (boiling point ) Antimony pentafluoride.