Four ternary silicides in the La–Ni–Si system: from polyanionic layers to frameworks

Abstract

The central part of the La–Ni–Si system has been investigated at 800 °C by means of single crystal X-ray diffraction, microscopy and analytical microprobe techniques. The result led to the identification of four new metal-rich silicides: LaNi₂Si (Rm, a = 4.0263(3) Å, c = 15.066(2) Å, Z = 3), La₂Ni₃Si₂ (P2₁/c, a = 6.8789(7) Å, b = 6.2167(3) Å, c = 12.214(1) Å, β = 90.92(1), Z = 4), La₃Ni₃Si₂ (Pnma, a = 7.501(2) Å, b = 14.316(4) Å, c = 6.149(2) Å, Z = 4), La₆Ni₇Si₄ (Pbcm, a = 6.066(1) Å, b = 7.488(1) Å, c = 29.682(5) Å, Z = 4). LaNi₂Si belongs to the SrCu₂Ga structure type, which is not represented among silicides, while La₂Ni₃Si₂ crystallizes in its own structure type. Both compounds feature layered polyanionic motifs consisting of Ni and Si, which are separated by La. Instead, La₆Ni₇Si₄ and La₃Ni₃Si₂ are characterized by polyanionic networks. The former compound belongs to the Pr₆Ni₇Si₄ structure type, with only two other representatives (Ce and Nd); the latter has been observed only with Rh and Ir. The two structures reveal close structural relationships having multiple identical slabs. Tight-binding electronic structure calculations by linear muffin-tin-orbital methods were performed for LaNi₂Si, La₂Ni₃Si₂ and La₃Ni₃Si₂ to gain insights into their structure–bonding relationships. Their band structures suggest a metallic character for all compounds. The overall crystal orbital Hamilton populations are dominated by polar Ni–Si bonds, though homoatomic Ni–Ni and La–Ni(Si) bond contributions are not negligible. The variety of bonding patterns may serve as a logical explanation for the number of discovered compounds in this system as well as for the diversity of the observed structures.