The type II clathrate Na22Si136 is prepared by the thermal decomposition of NaSi. Thermal analysis indicates this phase is metastable yet has a relatively high decomposition temperature. Rietveld analysis indicates that Na in the larger Si28 cage is shifted off-center, analogous to observations in some type I clathrates. Temperature-dependent electrical and thermal transport properties are reported for Na22Si136, for which the spark plasma sintering technique was found to be effective in achieving intergrain sintering in the consolidated specimen. The potential that type II clathrate materials possess for thermoelectric applications is discussed.
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Beekman et al. (2009) studied this question.