La-Ni-Si: A Gold Mine with a Diamond

The La-poor part of the ternary La-Ni-Si system has been explored leading to the discovery and structural characterization of four new polar intermetallic compounds. LaNi₅Si₂ [BaAu₅Ga₂ type, oP64, space group Pnma, a = 7.8223(7) Å, b = 6.3894(6) Å, c = 17.843(2) Å, V = 891.8(2) ų, Z = 8] features a diamond (lonsdaleite)-like homoatomic Ni framework and is the first Ni representative of a larger family of compounds typically formed by aurides. La₂Ni₈Si₃ [Eu₂Ni₈Si₃ type, tP52, P4₂/nmc, a = 10.0278(3) Å, c = 7.5047(4) Å, V = 754.65(6) ų, Z = 4] is characterized by homoatomic Ni₄ tetrahedra and rectangles. LaNi₅Si₃ [SrNi₅P₃ type, oS36, Cmcm, a = 3.722(2) Å, b = 11.759(5) Å, c = 11.622(3) Å, V = 508.7(3) ų, Z = 4] is governed by extensive heteroatomic bonding and characterized by homopolyhedral packing. La₃Ni₄Si₂ [Ce₃Ni₄Si₂ type, mC36, C2/c, a = 15.819(1) Å, b = 6.0068(5) Å, c = 7.4918(6) Å, β = 103.163(5)°, V = 693.17(10) ų, Z = 4] is a new member of homologous series that includes La₃Ni₃Si₂ and La₃Ni_3.5Si₂. We conclude that the similar radii and electronegativities of Ni and Si are the reason for the incredible diversity of compositions and structures in this system.