Thermoelectric properties and microstructure of Ba8Al14Si31 and EuBa7Al13Si33

Powder samples of the type I clathrate phases, with the proposed stoichiometry Ba8Al16Si30 and Eu2Ba6Al16Si30, 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 Ba8Al16Si30 sample was determined from microprobe data to be Ba8Al14Si31, close to the deficient framework solid solution of the general formula, Ba8AlxSi42-3/4x[](4-1/4x) (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 EuBa7Al13Si32. Both phases show conductivity typical of heavily doped semiconductors with negative Seebeck coefficients. The highest Seebeck coefficients are - 99 and - 71 mu V/K at 1192 K for Ba8Al14Si31 and EuBa7Al13Si33, respectively. The maximum zT is 0.34 at 1150 K and 0.22 at 1165 K for Ba8Al14Si31 and EuBa7Al13Si33, respectively.