Y4Ir9Si5, a new substitution variant of the Ce2Ni7 type and isotypes of other members of the R2+nT3+3nM1+2n structure series

Abstract

Y4Irg+xSi5_ x, x=0.12 (2), Mr= 2226 for Y4IrgSi 5, hP36, P63/mmc, a = 5.4894 (2), c = 22.6054 (9) A, V= 589-93 (6) A 3, Z= 2, Dx= 12.53Mgm -3 (Y4Ir9Sis), MoKa, :t = 0. 71073 A, :t =127.4mm -~, F(000)=1838, T=293K, wR= 0.061 for 308 independent reflections. The Y4IrgSi 5 type is a site-occupation variant of the Ce2Ni 7 type. It is a new member (n- 2) of the hexagonal branch of the structure series R2+nT3+3nll/11+2n , grouping types where ternary Laves-type slabs are intergrown with ternary CeCo3B2-type slabs. ErRh2Si, ErRh2Ge, YIr2Si and YRh2Ge were found to crystallize with the YRh2Si type (site-occupation variant of the CeNi 3 type), Er4RhgGe 5 and Y4RhgGe5 with the Y4RhgSi 5 type (site-occupation variant of the Gd2Co 7 type). The former structure type belongs to the hexagonal and the latter to the rhombohedral branch of the same structure series. Introduction. The well known Laves types and the CaCu 5 type are widely spread among binary com- pounds. In several R-T systems (R = rare-earth ele- ment, T= transition metal), structures where one Laves-type slab (R2T4) is intergrown with n CaCu:type slabs (RT 5) have been found at intermediate com- positions. The R2+nT4+sn structure series grouping these structures can be subdivided into two branches depend- ing on the stacking of the Laves-type slabs. In the rhombohedral branch the Laves-type slabs are stacked as in the cubic Laves-type MgCu 2 and in the hexagonal branch the same slabs are stacked as in the hexagonal MgZn 2 type (Parth6 & Lemaire, 1975). Simple ordered substitution variants with composition R2T3M (M = main-group element) are known for both Laves types: Mg2Cu3Si (Witte, 1939) with MgZn2-type stacking and Mg2Ni3Si (Nortus, Eriksson, Gtthe & Werner, 1985)or Y2RhaGe (Cenzual, Chabot & Partht, 1987) with MgCu2-type stacking. CeCoaB 2 (Kuz'ma, Kripyakevich & Bilonizhko, 1969) is a ternary ordered substitution variant of CaCu 5 with composition R T3M 2. Three ternary structure series can then be imagined where one or both kinds of slabs are ternary. The members of these series are substitution variants of the members of the binary R2÷nT4÷5~ structure series. In reality, several borides have been found to crystallize