“…The lanthanides in such a configuration are Eu 2+ , Gd 3+ , and Tb 4+ ; and the actinide ions include Am 2+ , Cm 3+ , and Bk 4+ . Early arguments were focused on the Gd 3+ ion since that the ground-state crystal-field splitting observed in EPR experiments was less than 0.5 cm À1 [63] and could not be interpreted satisfactorily by the crystal-field theory. A series of mechanisms were considered but failed to provide a consistent interpretation [62,64,65].…”