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Synthesis, structure, thermoelectric properties, and band gaps of alkali metal containing type I clathrates: A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38

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A series of alkali metal containing compounds with type I clathrate structure, A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38, were synthesized and characterized. Room temperature lattice parameters of A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38 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, Pm3Ì...n) was confirmed for all phases, and in the case of K8Al8Si38 and K8Ga8Si38, 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 K8Al8Si38 and K8Ga8Si38 being n-type semiconductors, while Rb8Ga8Si38 and Cs8Ga8Si38 were p-type semiconductors. K8Al8Si38 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. K8Ga8Si38 provides the lowest thermal conductivity, below 0.5 W/Km, comparable to the type I clathrate with heavy elements such as Ba8Ga16Ge30. Surface photovoltage spectroscopy on films shows that these compounds are semiconductors with band gaps in the range 1.14 to 1.40 eV.

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