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.

We synthesized a Si-based clathrate, composed entirely of Earth abundant elements, and using ab initio calculations and spectroscopic and Hall mobility measurement showed that it is a promising material for solar energy conversion.

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To investigate the quality of each sample after hot pressing, synchrotron powder XRD (SPXRD) was utilized. Representative patterns of the x p = 1, 2, 5, and 6 samples of Eu 11 Cd 6-x Zn x Sb 12 solid solutions are shown in Figure S1. The best profile matching was achieved by using the crystal structures solved for crystals of each reaction. The corresponding calculated patterns are shown as orange dashed lines overlaid on each observed pattern, shown in black. This profile matching results in the best figures of merit when the main phase with 11-6-12 structure and the minor phase related to the 9-4-9 structure were used in refinement. Samples with x p = 3 and 4 showed high volume fractions of 9-4-9 structure resulting to not be considered in this study. Lebail profile matching on the powder patterns of the other samples with x p = 1, 2, 5, and 6 showed a minor contribution from 9-4-9 structure.Zn Lα X-ray maps given by EMPA on single crystals and pressed pellets are provided in Figure S2. The average compositions with standard deviations in parentheses are also listed below the corresponding images. There is good agreement between the compositions obtained from single crystal X-ray diffraction and from EMPA. Zn distribution on all samples show homogenous samples except in x p = 2 pellet that two regions of high Zn and low Zn contents are making larger standard deviations.

Ba1.9Ca2.4Mg9.7Si7 was grown as large crystals from the reaction of silicon with barium and calcium in Mg/Al flux. This compound is a charge-balanced Zintl phase with the Zr2Fe12P7 structure type in hexagonal space group P6̅ (a = 11.196(2) Å, c = 4.4595(9) Å); barium occupies the Zr sites, the lighter alkaline earths (Mg, Ca) mix on the Fe sites, and silicon occupies the phosphorus sites. An isostructural germanide (Ba1.2Sr0.9Mg11.9Ge7, a = 11.064(1) Å, c = 4.3709(6) Å)) was similarly synthesized from reactions of germanium with barium and strontium in Mg/Al flux. Density of states calculations indicate these phases are semimetals, in agreement with their charge-balanced nature. Measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient were carried out from 300–1000 K to investigate the thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7. This compound is an n-type semimetal with low thermal conductivity (2 W/(m·K)), moderate Seebeck coefficient (−200 μV/K at 900 K), and a thermoelectric figure of merit ZT of 0.35 at 900 K.

The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses a high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K guest atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8–x Ba x Al8+x Si38–x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3̅n), confirmed by X-ray diffraction. The guest atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-containing samples were characterized from 2 to 300 K. The K–Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the temperature range of 323 to 873 K. Thermal conductivity from 323 to 873 K was estimated from the Wiedemann–Franz relation and lattice thermal conductivity extrapolation from 300 to 873 K. K8–x Ba x Al8+x Si38–x (x = 1, 1.5) synthesized with Al deficiency showed enhanced electrical conductivity, and the absolute Seebeck coefficients decrease with the increased carrier concentration. When x = 2, the Al content increases toward the electron balanced composition, and the electrical resistivity increases with the decreasing charge carrier concentration. Overall, K6.5Ba1.5Al9Si37 achieves an enhanced zT of 0.4 at 873 K.