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

The quaternary intermetallics Ce2CoGa9Ge2, Ce2NiGa9Ge2, and Sm2NiGa9Ge2 were prepared by reacting elemental metals in excess of gallium at 850 degrees C. The title compounds crystallize in the tetragonal space group P4/nmm in the Sm2Ni(Si(1-x)Ni(x))Al4Si6 structure type with cell parameters a = 5.9582(5) A, c = 15.0137(18) A, and a = 5.9082(17) A, c = 14.919(6) A, 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 Ce(3+) 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.