La n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)- z : 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 12.5 Ni 28.0 Si 18.3 ( n = 3; a = 28.8686(8), c = 4.0737(2) Å, Z = 3), La 22.1 Ni 39.0 Si 27.8 ( n = 4; a = 20.9340(6), c = 4.1245(2) Å, Z = 1), La 32.9 Ni 49.8 Si 39.3 ( n = 5; a = 24.946(1), c = 4.1471(5) Å, Z = 1), and La 44.8 Ni 66.1 Si 53.4 ( n = 6; a = 28.995(5), c = 4.158(1) Å, Z = 1) crystallize in the hexagonal space group P 6 3 / m and can be generalized according to La n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)- z with n = 3-6. Their crystal structures are based on AlB 2 -type building blocks, fused La-centered Ni 6 Si 6 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. La n ( n +1)+ x Ni n ( n +5)+ y Si ( n +1)( n +2)- z can be considered as a mirror series to the La-rich La ( n +1)( n +2) Ni n ( n -1)+2 Si 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 the first system where two such analogous series have been observed.