Rare earth-rich compounds RE9TMg4 (RE = Y, Dy-Tm, Lu; T = Ru, Rh, Os, Ir) with an ordered Co2Al5-type structure

Abstract

The rare earth-rich intermetallic phases RE9TMg4 (RE = Y, Dy-Tm, Lu; T = Ru, Rh, Os, Ir) were synthesized by induction melting of the elements using sealed niobium ampoules as crucible material. The melted samples were additionally annealed in muffle furnaces and subsequently characterized by X-ray powder diffraction. The RE9TMg4 compounds adopt an ordered Co2Al5 type structure, space group P63/mmc. Four structures were refined from single-crystal X-ray diffractometer data: a = 953.71(5), c = 968.41(5) pm, wR2 = 0.00273, 603 F2 values, 21 parameters for Tm8.76RuMg4.24; a = 958.37(5), c = 975.66(5), wR2 = 0.00384, 661 F2 values, 20 parameters for Dy9OsMg4; a = 943.70(5), c = 967.91(5) pm, wR2 = 0.00430, 592 F2 values, 21 parameters for Tm8.74OsMg4.26; a = 968.09(5), c = 978.25(5) pm, wR2 = 0.0439, 623 F2 values, 21 parameters for Y9.18IrMg3.82. The compounds are prone to small homogeneity ranges (RE/Mg mixing). The transition metal atoms have tricapped trigonal prismatic rare earth coordination. These T@RE9 units (TP) are condensed with empty RE6 octahedra (O) via common triangular faces forming infinite strands with a sequence –TP–O–O–. These strands show the motif of hexagonal rod packing and they are separated by chains of edge- and corner-sharing tetrahedra. The magnesium substructures in the hexagonal Laves phase YMg2 and the prototype Y9CoMg4 are structurally closely related. Charge transfer trends, electronic band structures and bonding properties were studied within DFT. The resulting picture is that cobalt brings covalent character by reducing the overall charge transfer and modifies the Laves phase YMg2 by providing larger localization in the density of states. The Y–Co bonding in Y9CoMg4 prevails while weakening the Y–Mg bonds. The investigations of the magnetic properties of selected RE9TMg4 compounds revealed Pauli paramagnetic behavior for Y9CoMg4, Y9OsMg4 and Y9IrMg4. A ferromagnetic ground state with Curie temperatures of 46.0 and 47.6 K was observed for Dy9RuMg4 and Dy9OsMg4, respectively. Ho9RuMg4, Ho9OsMg4 and Tm9OsMg4 reveal antiferromagnetic ordering with Neél temperatures below 20 K.