The nucleation energy of a series of LaxFeySbz modulated elemental reactants was measured as a function of the Fe/Sb ratio over a large composition range while holding the La content constant. The nucleation energy of the ternary compound La0.5Fe4Sb12 with the skutterudite crystal structure was found to depend very strongly on the Fe/Sb ratio in the modulated elemental reactant, with a higher nucleation energy as the Fe/Sb ratio is moved away from the 1:3 stoichiometric value. When the results of this study are compared with those from FeySbz modulated reactants, the addition of lanthanum was found to suppress the nucleation of FeSb2, thereby broadening the Fe/Sb composition range in which the ternary skutterudite compound LaxFe4Sb12 nucleates. This suppression of nucleation of a binary phase on addition of a ternary component to an amorphous intermediate is in agreement with theoretical arguments. The observed suppression of nucleation also provides rational for the observed nucleation of metastable ternary and higher-order compounds from homogeneous amorphous reactants.