Lanthanum Nickel Silicides with the General Formula La(n+1)(n+2)Nin(n-1)+2Sin(n+1) and Other Series of Hexagonal Structures with Metal:Metalloid Ratios Close to 2:1

The closely related crystal structures of the compounds La(6)Ni(2)Si(3) (La(12)Ni(4)Si(6), n = 2) and La(5)Ni(2)Si(3) (La(20)Ni(8)Si(12), n = 3) as well as those of the new compounds La(15)Ni(7)Si(10) (La(30)Ni(14)Si(20), n = 4) and La(21)Ni(11)Si(15) (La(42)Ni(22)Si(30), n = 5) were determined from single-crystal X-ray diffractometer data. In these hexagonal silicides (space group P6(3)/m) almost all atoms are situated on mirror planes, which extend perpendicular to the short c axes. The only exceptions are nickel atoms located on the 6(3) axes, which show some disorder resulting in the exact compositions La(6)Ni(1.54(2))Si(3) [a = 1224.3(1) pm, c = 438.3(1) pm, Z = 2, Ce(6)Ni(2)Si(3) type structure], La(5)Ni(1.75(1))Si(3) [a = 1624.4(3) pm, c = 434.2(2) pm, Z = 4, Ce(5)Ni(2)Si(3) type], La(15)Ni(6.62(2))Si(10) [a = 2021.2(3) pm, c = 435.1(1) pm, Z = 2, Pr(15)Ni(7)Si(10) type], and La(21)Ni(10.49(3))Si(15) [a = 2427.7(3) pm, c = 435.2(1) pm, Z = 2, a new structure type]. All of these structures have building elements which resemble the AlB(2) type structure. In these the lanthanum atoms correspond to the aluminum atoms of AlB(2), while the nickel and silicon atoms occupy the boron positions in an ordered manner. Other series of ternary hexagonal lanthanoid and actinoid transition metal silicides and phosphides, e.g., the series with the general formula R(n)()(()(n)()(-)(1))Ni(()(n)()(+1)()(n)()(+2))P(n)()(()(n)()(+1)+1) (R = Th, U), are briefly reviewed, and the systematics in the space groups and the diffraction patterns of these compounds are pointed out.