Lan(n+1)+xNin(n+5)+ySi(n+1)(n+2)-z: A Symmetric Mirror Homologous Series in the La-Ni-Si System

A homologous series La n(n+1)+x Ni n(n+5)+y Si(n+1)(n+2)-z has beenobserved inthe La-Ni-Si representing a symmetric Ni-rich mirrorto the La(n+1)(n+2)Ni n(n-1)+2Si n(n+1).A series of fourhomologous silicides have been discovered duringsystematic explorations in the central part of the La-Ni-Sisystem at 1000 & DEG;C. All compounds La12.5Ni28.0Si18.3 (n = 3; a = 28.8686(8), c = 4.0737(2) & ANGS;, Z = 3), La22.1Ni39.0Si27.8 (n = 4; a = 20.9340(6), c = 4.1245(2) & ANGS;, Z = 1), La32.9Ni49.8Si39.3 (n = 5; a = 24.946(1), c = 4.1471(5) & ANGS;, Z = 1), and La44.8Ni66.1Si53.4 (n =6; a = 28.995(5), c = 4.158(1) & ANGS;, Z = 1) crystallize in the hexagonal space group P6(3)/m and can be generalizedaccording to La n(n+1)+x Ni n(n+5)+y Si(n+1)(n+2)-z with n = 3-6. Their crystalstructures are based on AlB2-type building blocks, fusedLa-centered Ni6Si6 hexagonal prisms, yieldinglarger oligomeric equilateral domains with the edge size equal to n. The domains extend along the c axisand show checkered ordering of the cationic and anionic parts, whileall their atoms are located on mirror planes. La n(n+1)+x Ni n(n+5)+y Si(n+1)(n+2)-z can beconsidered as a mirror series to the La-rich La(n+1)(n+2)Ni n(n-1)+2Si n(n+1), where an exchange of the formal cationic and anionic sites,i.e., La and Si, occurs. The La-Ni-Si system is thefirst system where two such analogous series have been observed.