Synthesis, Structure, Thermoelectric Properties, and Band Gaps of Alkali Metal Containing Type I Clathrates: A₈Ga₈Si₃₈ (A = K, Rb, Cs) and K₈Al₈Si₃₈

A series of alkali metal containing compounds with type I clathrate structure, A 8 Ga 8 Si 38 (A = K, Rb, Cs) and K 8 Al 8 Si 38, were synthesized and characterized. Room temperature lattice parameters of A 8 Ga 8 Si 38 (A = K, Rb, Cs) and K 8 Al 8 Si 38 were determined to be 10.424916(10), 10.470174(13), 10.535069(15), and 10.48071(2) Å, respectively. The type I clathrate structure (cubic, Pm 3̅ n ) was confirmed for all phases, and in the case of K 8 Al 8 Si 38 and K 8 Ga 8 Si 38, the structures were also refined using synchrotron powder diffraction data. The samples were consolidated by Spark Plasma Sintering (SPS) for thermoelectric property characterization. Electrical resistivity was measured by four probe AC transport method in the temperature range of 30 to 300 K. Seebeck measurements from 2 to 300 K were consistent with K 8 Al 8 Si 38 and K 8 Ga 8 Si 38 being n-type semiconductors, while Rb 8 Ga 8 Si 38 and Cs 8 Ga 8 Si 38 were p-type semiconductors. K 8 Al 8 Si 38 shows the lowest electrical resistivity and the highest Seebeck coefficient. This phase also showed the largest thermal conductivity at room temperature of ∼1.77 W/Km. K 8 Ga 8 Si 38 provides the lowest thermal conductivity, below 0.5 W/Km, comparable to the type I clathrate with heavy elements such as Ba 8 Ga 16 Ge 30 . Surface photovoltage spectroscopy on films shows that these compounds are semiconductors with band gaps in the range 1.14 to 1.40 eV.