Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.

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 °C forming kinetically stable "filled" skutterudites of general formula M 1-x Fe 4 Sb 12 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 (∼40 Å), the thermodynamically stable binary compound FeSb 2 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.

A series of samples ((AB)(x)(CD)(y))(z) were prepared containing both short repeat units (AB and CD) and long repeat units ((AB)(x)(CD)(y)), where the short repeat units were designed to have the composition appropriate to form square M(4)Sb(12) skutterudites (M = Fe, Co, or Ir; square = vacancy, La, or Y). X-ray diffraction and reflectivity were used to follow the evolution of the films from amorphous, layered materials to crystalline skutterudite superlattices as a function of annealing temperature and time. In all cases, the short repeat units interdiffused and crystallized the expected skutterudite, while the long repeat period persisted after annealing. The skutterudites crystallize with random crystallographic orientation with respect to the substrate. The observed splitting of the peaks in the high-angle diffraction data from the IrSb(3)/CoSb(3) sample indicates the formation of a novel superlattice structure with each grain having a random crystallographic orientation of the skutterudite lattice with respect to the superlattice direction.

The evolution of modulated elemental reactants containing alternating layers of carbon and Ti, W, or Mo was studied using differential scanning calorimetry and X-ray diffraction. TiC crystallized at 350 °C from a modulated reactant with a composition near 1:1 with respect to Ti and C. Mo 2 C and W 2 C nucleated at 500 and 600 °C, respectively, from modulated reactants with compositions close to their stoichiometry. No exotherms were observed in samples that had compositions that were not close to the stoichiometries of TiC, Mo 2 C, or W 2 C.

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