TERNARY RARE-EARTH TRANSITION-METAL ZINC-COMPOUNDS RT2ZN20 WITH T=FE,RU, CO, RH, AND NI

Forty eight new compounds RT2Zn20 were prepared by annealing cold-pres sed pellets of the elemental components in an argon atmosphere. They c rystallize with the cubic CeCr2Al20 type structure (Fd (3) over bar m, Z = 8), which was refined from single-crystal diffractometer data of TbFe2Zn20 (a = 1411.1(1)pm), YRu2Zn20 (a = 1422.6(1) pm), DyRu2Zn20 (a = 1422.1(1)pm), GdCo2Zn20 (a = 1406.0(1) pm), DyRh2Zn20 (a = 1418.2(1 ) pm), and TmNi2Zn20 (a = 1401.6(1) pm) to conventional residuals vary ing between R = 0.011 and R = 0.024. The compounds have a tendency for twinning, thus mimicking hexagonal symmetry, with the cubic [111] axi s as the axis with the pseudohexagonal symmetry. Minor inconsistencies in the cell volumes of these compounds indicate slight deviations fro m the ideal composition. Nevertheless, the five atomic sites of this s tructure were found to be fully occupied within the error limits with the exception of one zinc site of TmNi2Zn20. The coordination for the site of the rare earth atoms is a Frank-Kasper polyhedron with coordin ation number (CN) 16. The transition metal atoms occupy a site with ic osahedral zinc coordination (CN 12). Two of the three zinc sites are i n pentagonal prismatic coordination of zinc atoms, capped by rare eart h and/or transition metal atoms (CN 12), while the third zinc site has 12 zinc neighbors forming a hexagonal prism, which is capped by two r are earth atoms (CN 14).