Polygallide RE₂MGa₉Ge₂ (RE = Ce, Sm; M = Ni, Co) phases grown in molten gallium

The quaternary intermetallics Ce₂CoGa₉Ge₂, Ce₂NiGa₉Ge₂, and Sm₂NiGa ₉Ge₂ were prepared by reacting elemental metals in excess of gallium at 850°C. The title compounds crystallize in the tetragonal space group P4/nmm in the Sm₂Ni(Si_1-xNi_x)Al ₄Si₆ structure type with cell parameters a = 5.9582(5) Å, c = 15.0137(18) Å, and a = 5.9082(17) Å, c = 14.919(6) Å, Z = 2, for Ce₂CoGa₉Ge₂ and Sm ₂NiGa₉Ge₂, respectively. The structures are composed of covalently bonded three-dimensional networks of [CoGa ₉Ge₂] in which the rare-earth metals fill the voids forming a 2D square net. The structures of RE₂MGa₉Ge ₂ are Ga-rich and possess extensive Ga-Ga bonding even though the Ga atoms do not form a network on their own. Magnetic susceptibility measurements for Ce₂CoGa₉Ge₂ and Ce₂NiGa ₉Ge₂ show Curie-Weiss paramagnetism, consistent with presence of Ce³⁺ ions. Magnetocrystalline anisotropy was observed for Ce₂NiGa₉Ge₂, with the magnetically easy axis lying along the [001] crystallographic direction. A transition to an antiferromagnetic state was observed below 4 K in the easy direction of magnetization. In the magnetically hard direction of the basal plane, paramagnetic behavior was observed down to 1.8 K.