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

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. Na₈Al_xSi_46–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.