Thermoelectric Properties and Microstructure of Ba8Al14Si31 and EuBa7Al13Si33

Powder samples of the type I clathrate phases, with the proposed stoichiometry Ba 8 Al 16 Si 30 and Eu 2 Ba 6 Al 16 Si 30, were synthesized using direct reaction of stoichiometric amounts of the elements. Rietveld refinement of powder X-ray diffraction data is consistent with the clathrate type I structure. Composition, microstructure, and thermoelectric property measurements were made on hot-pressed pellets. Stoichiometry of the Ba 8 Al 16 Si 30 sample was determined from microprobe data to be Ba 8 Al 14 Si 31, close to the deficient framework solid solution of the general formula, Ba 8 Al x Si 42 - 3/4 x [] 4 - 1/4 x ( x = 14; [], open square indicates lattice defect). In the case of the rare-earth-substituted compound, microprobe analysis of the microstructure indicates that it is not single phase, but contains multiple components, most of the general clathrate stoichiometry. The majority phase has the stoichiometry EuBa 7 Al 13 Si 32 . Both phases show conductivity typical of heavily doped semiconductors with negative Seebeck coefficients. The highest Seebeck coefficients are − 99 and −71 μV/K at 1192 K for Ba 8 Al 14 Si 31 and EuBa 7 Al 13 Si 33, respectively. The maximum zT is 0.34 at 1150 K and 0.22 at 1165 K for Ba 8 Al 14 Si 31 and EuBa 7 Al 13 Si 33, respectively.