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) Å3, 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) Å3, 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) Å3, 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) Å3, Z = 4] is a new member of homologous series that includes La3Ni3Si2 and La3Ni3.5Si2. 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.