A new metastable binary compound and a series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been prepared through controlled crystallization of amorphous reaction intermediates formed by low-temperature interdiffusion of modulated elemental reactants. Discussed in this paper are La1-xFe4Sb12, FeSb3, Hf1-xFe4Sb12, and Y1-xFe4Sb12. The amorphous reaction intermediate for each system crystallizes exothermically near 200 degrees C forming the desired skutterudite structure. At temperatures above 500 degrees C, they decompose exothermically forming a thermodynamically more stable mixture of binary compounds and elemental components. We propose that the desired compounds nucleate from the amorphous precursor because slow solid state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. This general synthetic approach to prepare new metastable compounds gives control of both the composition and structure of the product compounds.