Rows of corner- and face-sharing Mg₄ tetrahedra arranged as hexagonal rod packing in the hexagonal Laves phases REMg₂ and the rare earth-rich phases RE₉CoMg₄ (RE=Y, Dy-Tm, Lu)

Six new rare earth metal-rich intermetallic compounds RE₉CoMg₄ with RE=Y, Dy, Ho, Er, Tm and Lu were synthesized by induction-melting the elements in sealed niobium ampoules followed by annealing in muffle furnaces. The structures of Y₉CoMg₄ and Tm_8.56CoMg_4.44 were refined from single-crystal X-ray diffraction data: P6₃/mmc, a=965.65(6), c=971.07(5) pm, wR2=0.0599, 614 F² values, 20 variables for Y₉CoMg₄ and a=945.20(4), c=953.11(5) pm, wR2=0.0358, 585 F² values, 21 variables for Tm_8.56CoMg_4.44 (a small homogeneity range results from Tm/Mg mixing). The RE₉CoMg₄ phases crystallize with a coloring variant of the aristotype Co₂Al₅. The striking structural motif is a hexagonal rod packing of rows of corner- and face-sharing tetrahedral Mg₄ clusters with Mg-Mg distances ranging from 304 to 317 pm in Y₉CoMg₄. These rows are similar to the hexagonal Laves phases REMg₂. The space between the rows is filled with rows of face-sharing Co rate at Y₆ trigonal prisms (TP) and empty Y₆ octahedra (O) in the sequence -TP-O-O-. The many isopointal coloring variants of the aristotype Co₂Al₅ are briefly discussed.