Precursor Routes to Complex Ternary Intermetallics: Single-Crystal and Microcrystalline Preparation of Clathrate-I Na₈Al₈Si₃₈ from NaSi + NaAlSi

Abstract

Single crystals of the ternary clathrate-I Na8Al8Si38 were synthesized by kinetically controlled thermal decomposition (KCTD), and microcrystalline Na8Al8Si38 was synthesized by spark plasma sintering (SPS) using a NaSi + NaAlSi mixture as the precursor. Na8AlxSi46-x compositions with x ≤ 8 were also synthesized by SPS from precursor mixtures of different ratios. The crystal structure of Na8Al8Si38 was investigated using both Rietveld and single-crystal refinements. Temperature-dependent transport and UV/vis measurements were employed in the characterization of Na8Al8Si38, with diffuse-reflectance measurement indicating an indirect optical gap of 0.64 eV. Our results indicate that, when more than one precursor is used, both SPS and KCTD are effective methods for the synthesis of multinary inorganic phases that are not easily accessible by traditional solid-state synthesis or crystal growth techniques.

Earth Abundant Element Type I Clathrate Phases

“…In these type I clathrates, the group 13 element favors the 6 c framework site with a small amount on the 24 k site in the ratio of approximately 5:3, when A = K, Rb, Cs. This is consistent with experimental observations for these alkali metal clathrates [7,8,9,11,13]. …”

“…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]. …”

“…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.…”

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).…”

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization

“…Recently, a thermal decomposition at lower temperatures was employed for the preparation of Na 24- x (Si y Ge 1- y ) 136 and proved to be useful for the synthesis of silicon clathrates [20]. Extensions of the SPS and KCTD approaches have also recently reported, including use of multi-phase precursors or simultaneous ion-exchange and electrochemical reactions to prepare ternary or quaternary clathrates [21,22,23]. …”