The Synthesis of Metastable Skutterudites and Crystalline Superlattices

Abstract

We have used controlled crystallization of elementally modulated reactants to prepare a series of kinetically stable, crystalline skutterudites (M'xM4Sb12 where M' = vacancy, RE, Hf,…; M = Ni, Fe, Co) and crystalline superlattices composed of promising thermoelectric materials. For the bulk synthesis of skutterudites, low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150°C. Nucleation of the skutterudite structure occurs with at large exotherm on annealing at temperatures below 200°C regardless of the ternary metal. All of the metastable ternary compounds and the new metastable binary compounds were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 250°C for the binary compound NiSb3 to above 550°C for the rare earth containing cobalt compounds. The occupation of the ternary site was found to depend on the composition of the initial reactant and was varied from 0 to 1. Full occupancy typically required an excess of the filling cation. The lattice parameters of the compounds prepared at low temperatures are distinctly smaller than those prepared using traditional synthetic approaches. High temperature annealing converts the lattice parameters of the low temperature compounds to those prepared at higher temperatures using traditional synthetic approaches. Diffraction patterns of crystalline superlattices containing skutterudites prepared using elementally modulated reactants show splitting of high angle diffraction maxima as well as the presence of the expected low angle diffraction pattern from a supperlattice. The skutterudite superlattices are stable with respect to low temperature annealing.

Synthesis and Properties of CexCo4Ge6Se6

“…Table shows the compositions of cerium-containing samples in which single-phase skutterudites were formed. All Ce-filled samples that formed single-phase skutterudites had exotherm peak temperatures of approximately 440–460 °C, somewhat higher than the unfilled samples as observed previously in comparable situations . For the samples that formed only the pure skutterudite phase, the DSC exotherm peak temperatures increased linearly with the increase in the amount of cerium (see Figure ).…”

“…On the other hand, a much greater amount of Ce was able to be incorporated into the CoSb 3 lattice using the thin-film synthesis method that we used . Because we were unable to include more than ∼20% cerium into our low-temperature samples, another potential explanation is needed.…”

Synthesis of Crystalline Skutterudite Superlattices Using the Modulated Elemental Reactant Method

“…We have used the modulated elemental reactant synthesis technique to prepare a wide variety of phases with the skutterudite crystal structure. − This synthesis method involves sequentially depositing ultrathin elemental layers to minimize the diffusion distance. This allows solid-state reactions to take place at very low temperatures (below 250° for the skutterudites) due to the small diffusion distances required for the reactants to form homogeneous and amorphous intermediates.…”

“…Because the skutterudite crystal structure is cubic, and nucleates and grows from an amorphous intermediate, these phases crystallize with random orientation. Two new binary, and several new ternary “filled” skutterudite compounds with atoms occupying the large void in the center of the unit cell, have been prepared in this way. ,− …”