La–Ni–Si:\nA Gold Mine with a Diamond

The La-poor part of the ternary La–Ni–Si\nsystem has\nbeen explored leading to the discovery and structural characterization\nof four new polar intermetallic compounds. LaNi<sub>5</sub>Si<sub>2</sub> [BaAu<sub>5</sub>Ga<sub>2</sub> type, <i>oP</i>64, space group <i>Pnma</i>, <i>a</i> = 7.8223(7)\nÅ, <i>b</i> = 6.3894(6) Å, <i>c</i> =\n17.843(2) Å, <i>V</i> = 891.8(2) Å<sup>3</sup>, <i>Z</i> = 8] features a diamond (lonsdaleite)-like homoatomic\nNi framework and is the first Ni representative of a larger family\nof compounds typically formed by aurides. La<sub>2</sub>Ni<sub>8</sub>Si<sub>3</sub> [Eu<sub>2</sub>Ni<sub>8</sub>Si<sub>3</sub> type, <i>tP</i>52, <i>P</i>4<sub>2</sub>/<i>nmc</i>, <i>a</i> = 10.0278(3) Å, <i>c</i> = 7.5047(4)\nÅ, <i>V</i> = 754.65(6) Å<sup>3</sup>, <i>Z</i> = 4] is characterized by homoatomic Ni<sub>4</sub> tetrahedra\nand rectangles. LaNi<sub>5</sub>Si<sub>3</sub> [SrNi<sub>5</sub>P<sub>3</sub> type, <i>oS</i>36, <i>Cmcm</i>, <i>a</i> = 3.722(2) Å, <i>b</i> = 11.759(5) Å, <i>c</i> = 11.622(3) Å, <i>V</i> = 508.7(3) Å<sup>3</sup>, <i>Z</i> = 4] is governed by extensive heteroatomic\nbonding and characterized by homopolyhedral packing. La<sub>3</sub>Ni<sub>4</sub>Si<sub>2</sub> [Ce<sub>3</sub>Ni<sub>4</sub>Si<sub>2</sub> type, <i>mC</i>36, <i>C</i>2/<i>c</i>, <i>a</i> = 15.819(1) Å, <i>b</i> = 6.0068(5) Å, <i>c</i> = 7.4918(6) Å, β\n= 103.163(5)°, <i>V</i> = 693.17(10) Å<sup>3</sup>, <i>Z</i> = 4] is a new member of homologous series that\nincludes La<sub>3</sub>Ni<sub>3</sub>Si<sub>2</sub> and La<sub>3</sub>Ni<sub>3.5</sub>Si<sub>2</sub>. We conclude that the similar radii\nand electronegativities of Ni and Si are the reason for the incredible\ndiversity of compositions and structures in this system.