Polygallide RE<sub>2</sub>MGa<sub>9</sub>Ge<sub>2</sub> (RE = Ce, Sm; M = Ni, Co) Phases Grown in Molten Gallium
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The quaternary intermetallics Ce2CoGa9Ge2, Ce2NiGa9Ge2, and Sm2NiGa9Ge2 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(Si1-xNix)Al4Si6 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 Sm2NiGa9Ge2, respectively. The structures are composed of covalently bonded three-dimensional networks of [CoGa9Ge2] in which the rare-earth metals fill the voids forming a 2D square net. The structures of RE2MGa9Ge2 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 Ce2NiGa9Ge2 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.
