Forty eight new compounds RT2Zn20 were prepared by annealing cold-pressed pellets of the elemental components in an argon atmosphere. They crystallize with the cubic CeCr2Al20 type structure (Fd3̅̅m , Z = 8), which was refined from single-crystal diffractometer data of TbFeiZn20 (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 varying betw een R = 0.011 and R - 0.024. The com pounds have a tendency for tw inning, thus m im icking hexagonal sym metry, with the cubic [111] axis as the axis w ith the pseudohexagonal symmetry. M inor inconsistencies in the cell volum es of these com pounds indicate slight deviations from the ideal com position. N evertheless, the five atom ic sites of this structure w ere found to be fully occupied w ithin the error lim its w ith the exception of one zinc site of TmNi2Zn20. The coordination for the site of the rare earth atom s is a Frank-K asper polyhedron with coordination num ber (CN) 16. The transition metal atom s occupy a site w ith icosahedral zinc coordination (CN 12). Two of the three zinc sites are in pentagonal prism atic coordination of zinc atom s, capped by rare earth and/or transition metal atom s (CN 12), w hile the third zinc site has 12 zinc neighbors form ing a hexagonal prism , w hich is capped by tw o rare earth atom s (CN 14).
Emergence of A-Site Cation Order in the Small Rare-Earth Melilites SrREGa3O7 (RE = Dy–Lu, Y)
