Phase stability and chemical composition dependence of the thermoelectric properties of the type-I clathrate Ba8AlxSi46−x (8≤x≤15)

Phase stability of the type-I clathrate compound Ba8AlxSi46−x and the thermoelectric property dependence on chemical composition are presented. Polycrystalline samples were prepared by argon arc melting and annealing. Results of powder X-ray diffraction and electron microprobe analysis show that the type-I structure is formed without framework deficiency for 8≤x≤15. Lattice constant a increases linearly with the increase of x. Thermoelectric properties were measured for x=12, 14 and 15. The Seebeck coefficients are negative, with the absolute values increasing with x. The highest figure of merit zT=0.24 was observed for x=15 at T=1000 K, with carrier electron density n=3×1021 cm−3. A theoretical calculation based on the single parabolic band model reveals the optimum carrier concentration to be n∼4×1020 cm−3, where zT∼0.7 at T=1000 K is predicted.

Graphical AbstractBa8AlxSi46−x is found to crystallize in the type-I clathrate structure without framework deficiency for 8≤x ≤15. Thermoelectric figure of merit reaches zT=0.24 for x=15 at 1000 K, and calculation predicts zT∼0.7 for a lower carrier concentration.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Phase relation of the type-I clathrate Ba8AlxSi46−x is clarified. ► Thermoelectric performance becomes higher with the increase of Al content x. ► Low lattice thermal conductivity comparable to glasses is observed. ► Figure of merit zT=0.7 is predicted from calculation at the carrier concentration n∼4×1020 cm−3.