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

Abstract

Å, respectively. The type I clathrate structure (cubic, Pm3̅ 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.

Ab initiocomputational study on the lattice thermal conductivity of Zintl clathrates[Si19P4]Cl4andHärkönen1, Karttunen2 2016Phys. Rev. B11190View full textAdd to dashboardCite

“…This facilitates the comparative analysis of these materials because some quantities for them are identical due to symmetry reasons. Type-I Zintl clathrates A 8 [Al 8 Si 38 ] (A = Na, K, Rb, Cs) and A 8 [Ga 8 Si 38 ] (A = K, Rb, Cs) have been recently synthesized [39][40][41]. The thermoelectric properties of the synthesized silicon clathrates have also been investigated and they show rather low thermal conductivity (about 2 W/(m K) for Na 8 [Al 8 Si 38 ] and below 0.5 W/(m K) for K 8 [Ga 8 Si 38 ] at T = 300 K, polycrystalline samples).…”

Earth Abundant Element Type I Clathrate Phases

“…The experimental measurements of A 8 Ga 8 Si 38 follow the predicted trend, but are larger than the original calculations [13]. The values of the bandgap for K 8 Al 8 Si 38 and A 8 Ga 8 Si 38 (A = K, Rb, Cs) were determined from surface photovoltage spectroscopy [41,42] indicated as n-type and are in the range of 1.1–1.4 eV [13]. Na 8 Al 8 Si 38 indicated an optical bandgap of 0.64 eV [9].…”

“…With a higher level of theory and site preferences of E similar to that found experimentally, the bandgap of K 8 Al 8 Si 38 was found to be larger than 1 eV [11]. The experimental measurements of A 8 Ga 8 Si 38 follow the predicted trend, but are larger than the original calculations [13]. The values of the bandgap for K 8 Al 8 Si 38 and A 8 Ga 8 Si 38 (A = K, Rb, Cs) were determined from surface photovoltage spectroscopy [41,42] indicated as n-type and are in the range of 1.1–1.4 eV [13].…”

“…The thermal conductivity in all cases is low and the Seebeck coefficients are reasonable, but the electrical resistivity is too high in most cases. In the example of K 8 Al 8 Si 38 , it is apparent from the reported results that the precise stoichiometry greatly affects the electrical resistivity, as can be seen in Table 3 with the various entries [13,14,37] and for the one example that is slightly off stoichiometry, K 8 Al 7 Si 39 [38]. …”

“…The phases prepared to date are A 8 E 8 Si 38 (A = Na [9,10], K [7,10,11,12], Rb [8], Cs) [13,14] and have been shown experimentally to be semiconducting when the composition is near the expected; the E = In and A = Na, E = Ga phases have not been prepared. Table 1 provides the lattice parameters of the A 8 E 8 Si 38 compounds reported to date.…”