On the RE₂TiAl₃ (RE = Y, Gd-Tm, Lu) Series-The First Aluminum Representatives of the Rhombohedral Mg₂Ni₃Si Type Structure

Several ternary rare-earth metals containing titanium aluminum intermetallics in the RE2TiAl3 series (RE = Y, Gd-Lu) have been synthesized from the elements using arc-melting techniques. All compounds crystallize in the trigonal crystal system with rhombohedral space group R (3) over barm (Z = 3) and lattice parameters ranging between a = 582-570 and c = 1353-1358 pm. They adopt the Mg2Ni3Si-type structure, which is an ordered superstructure of the cubic Laves phase MgCu2 and has been observed for Al intermetallics for the first time. Tetrahedral [TiAl3] entities that are connected over all corners form a network where the empty [TiAl3] tetrahedra exhibit a full Ti/Al ordering based on the single crystal results. The Al atoms are arranged into 6(3) Kagome nets, while the Ti atoms connect these nets over the triangular units. In the cavities of this three-dimensional arrangement, the RE cations can be found forming a distorted diamond-type substructure. Magnetic measurements revealed that Y2TiAl3 and Lu2TiAl3 are Pauli paramagnetic substances, in line with the metallic character. The other compounds exhibit paramagnetism with antiferromagnetic ordering at a maximum Neel temperature of T-N = 26.1(1) K for Gd2TiAl3.