Rhombohedral Laves phases − the germanides RE ₂ Rh ₃ Ge ( RE = La, Nd, Dy, Tm, Yb, Lu) and the solid solutions Gd ₂ Rh ₃ Ge 1− x Ga x

Polycrystalline samples of the germanides RE 2 Rh 3 Ge with RE = La, Nd, Sm, Gd, Dy, Ho, Tm, Yb and Lu were synthesized from the elements by arc-melting or within sealed niobium ampoules followed by annealing in order to increase phase purity and crystallinity. The germanides with RE = La, Nd, Dy, Tm, Yb and Lu are reported here for the first time. The samples were characterized by X-ray powder diffraction. They are ternary ordered Laves phases and crystallize with the rhombohedrally distorted variant of the Mg 2 Ni 3 Si type, space group R <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mover accent="true"> <m:mn>3</m:mn> <m:mo>̅</m:mo> </m:mover> </m:mrow> </m:math> ̅3 m . In addition, the solid solutions Gd 2 Rh 3 Ge 1– x Ga x were studied and the structure of Gd 2 Rh 3 Ge 0.5 Ga 0.5 was refined from single-crystal X-ray diffractometer data: a = 558.34(9), c = 1,187.9(2) pm, w R 2 = 0.0350, 174 F 2 values and 11 variables. The rhombohedral distortion is caused by the rhodium-for-germanium (gallium) substitution within the network of condensed tetrahedra and avoids too short RE – RE distances. The RE 2 Rh 3 Ge germanides were studied with respect to their magnetic properties. La 2 Rh 3 Ge and Lu 2 Rh 3 Ge carry no permanent magnetic moments and behave as Pauli paramagnets. The other compounds are Curie-Weiss paramagnets with stable trivalent ground states. They are ordered ferromagnetically below Curie temperatures of 12.2(1) K (Nd), 57.6(1) K (Sm), 63.1(1) K (Gd), 22.3(1) K (Dy), 13.8(1) K (Ho) and 9.0(1) K (Tm). The 2 K magnetization isotherms classify these RE 2 Rh 3 Ge germanides as soft ferromagnets which show a tendency to saturation already at around 10 kOe. A switch in the magnetic ground state occurs for the solid solutions Gd 2 Rh 3 Ge 1– x Ga x , from antiferromagnetic Gd 2 Rh 3 Ga ( T N = 44.5(1) K) to ferromagnetic Gd 2 Rh 3 Ge with T C = 63.1(1) K with a monotonous increase of the magnetic ordering temperature.