Synthesis and Crystal Structures of BaLaSi₂ with cis-trans Si Chains and Ba₅LaSi₆ with Pentagonal Si Rings

Prismatic single crystals of novel compounds BaLaSi₂ and Ba₅LaSi₆ were synthesized from the elements with or without a Na flux. The crystal structures of the compounds were analyzed by X-ray diffraction. BaLaSi₂, containing cis-trans ¹_∞[Si] chains, crystallizes in an orthorhombic cell (a = 4.6414(2) Å, b = 14.8851(7) Å, c = 6.7519(5) Å; space group, Cmcm (No. 63), Z = 4) and is isotypic with the low-temperature phase of LaSi. The crystal structure of Ba₅LaSi₆ (a = 17.1447(5) Å, b = 4.8767(1) Å, and c = 17.9102(4) Å; space group Pnma (No. 62), Z = 4) is a new type containing isolated anionic groups (⁰[Si₅-Si]) of a pentagonal Si ring with a Si-Si stem. The electrical resistivities measured for the polycrystalline BaLaSi₂ and Ba₅LaSi₆ sintered samples were 0.31 and 0.48 mΩ·cm, respectively, at 300 K and increased with temperature. The Seebeck coefficients of BaLaSi₂ and Ba₅LaSi₆ were -7.6 and -11 μV·K^-1, respectively, at 296 K.