Synthesis of metastable post-transition-metal iron antimony skutterudites using he multilayer precursor method

Modulated elemental reactants were used to form amorphous ternary reaction intermediates of the desired compositions. The amorphous reaction intermediate for each system crystallizes exothermically below 200 degrees C forming kinetically stable "filled" skutterudites of general formula M1-xFe4Sb12 where M = Sn, Al, Ga, In, and Zn. These metastable compounds can only be prepared by controlling the reaction intermediates, avoiding the formation of more thermodynamically stable binary compounds. 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. If modulated elemental reactants are prepared with layer thicknesses above a critical value (similar to 40 Angstrom), the thermodynamically stable binary compound FeSb2 interfacially nucleates, preventing formation of the desired metastable ternary compounds. The use of low-angle X-ray diffraction and calorimetry provide the data necessary to develop an understanding of the reaction mechanism.