LANTHANUM NICKEL SILICIDES WITH THE GENERAL FORMULA LA(N-1)+2SIN(N+1)AND OTHER SERIES OF HEXAGONAL STRUCTURES WITH METAL(L)(N+2)NIN(N)METALLOID RATIOS CLOSE TO 2/1/

The closely related crystal structures of the compounds La6Ni2Si3 (La1 2Ni4Si6, n = 2) and La5Ni2Si3 (La20Ni8Si12, n = 3) as well as those of the new compounds La15Ni7Si10 (La30Ni14Si20, n = 4) and La21Ni11Si15 (La42Ni22Si30, n = 5) were determined from single-crystal X-ray diffra ctometer data. In these hexagonal silicides (space group P6(3)/m) almo st 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 composi tions La6Ni1.54(2)Si-3 [a = 1224.3(1) pm, c = 438.3(1) pm, Z = 2, Ce6N i2Si3 type structure], La5Ni1.75(1)Si-3 [a = 1624.4(3) pm, c = 434.2(2 ) pm, Z = 4, Ce5Ni2Si3 type], La15Ni6.62(2)Si-10 [a = 2021.2(3) pm, c = 435.1(1) pm, Z = 2, Pr15Ni7Si10 type], and La21Ni10.49(3)Si-15 [a = 2427.7(3) pm, c = 435.2(1) pm, Z = 2, a new structure type]. All of th ese structures have building elements; which resemble the AlB2 type st ructure. In these the lanthanum atoms correspond to the aluminum atoms of AlB2, while the nickel and silicon atoms occupy the boron position s in an ordered manner. Other series of ternary hexagonal lanthanoid a nd actinoid transition metal silicides and phosphides, e.g., the serie s with the general formula Rn(n-1)Ni(n+1(n+2))Pn(n+1+1) (R = Th, U), a re briefly reviewed, and the systematics in the space groups and the d iffraction patterns of these compounds are pointed out.