LaNiSi₍: A Symmetric Mirror Homologous Series in the La-Ni-Si System

A series of four homologous silicides have been discovered during systematic explorations in the central part of the La-Ni-Si system at 1000 °C. All compounds La_12.5Ni_28.0Si_18.3 (n = 3; a = 28.8686(8), c = 4.0737(2) Å, Z = 3), La_22.1Ni_39.0Si_27.8 (n = 4; a = 20.9340(6), c = 4.1245(2) Å, Z = 1), La_32.9Ni_49.8Si_39.3 (n = 5; a = 24.946(1), c = 4.1471(5) Å, Z = 1), and La_44.8Ni_66.1Si_53.4 (n = 6; a = 28.995(5), c = 4.158(1) Å, Z = 1) crystallize in the hexagonal space group P6₃/m and can be generalized according to LaNiSi₍ with n = 3-6. Their crystal structures are based on AlB₂-type building blocks, fused La-centered Ni₆Si₆ hexagonal prisms, yielding larger oligomeric equilateral domains with the edge size equal to n. The domains extend along the c axis and show checkered ordering of the cationic and anionic parts, while all their atoms are located on mirror planes. LaNiSi₍ can be considered as a mirror series to the La-rich La₍NiSi, where an exchange of the formal cationic and anionic sites, i.e., La and Si, occurs. The La-Ni-Si system is the first system where two such analogous series have been observed.