Structural chemistry and magnetic behavior of ternary gallides REAuxGa4−x, RE = La, Ce, Pr, Nd, and Sm

Two new series of ternary gallides with the chemical formulas (La, Ce, Pr, Nd, Sm)Au_xGa_4−x and (La, Ce, Pr, Nd, Sm)Au_1.5Ga_2.5 were synthesized from the elements by arc melting. From X-ray powder diffraction analysis the REAu_xGa_4−x series of compounds was found to be isotypic and crystallize with BaAl₄ type of structure; the homogeneity range at 600°C of each of the REAu_xGa_4−x phases was established, revealing remarkable deviations from Vegard's rule. At 600°C the REAu_xGa_4−x phases of the BaAl₄ type were observed to be in thermodynamic equilibrium with a structure variant crystallizing at the composition REAu_1.5Ga_2.5 and with a narrow homogeneous range. In case of PrAu_1.5Ga_2.5 the structure type was refined from X-ray single-crystal counter data (CaBe₂Ge₂ type, space group P4/nmm, R_w = 0.046). Gold and gallium atoms were generally found on separate crystallographic sites; however, a statistical distribution of 51% Au + 49% Ga was derived for the 2b sites. ¹⁹⁷Au Mössbauer spectroscopy confirmed the occupational mode of the gold atoms in CeAu_1.5Ga_2.5. For the BaAl₄-type phases, X-ray powder and Mössbauer data revealed preferential occupation of the 4e sites of I4/mmm by gold atoms; practically no Au was observed on the 4d sites. Magnetic susceptibilities were determined over a temperature range extending from 2 to 1100 K. Above liquid nitrogen temperatures the paramagnetic behavior of the (Ce, Pr, Nd)Au_xGa_4−x and the (Ce, Pr, Nd)Au_1.5Ga_2.5 compounds is characterized by magnetic moments close to the ideal trivalent rare earth values. Lanthanum compounds are diamagnetic, whereas SmAu_xGa_4−x alloys are characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets. At very low temperatures onset of ferromagnetic ordering is observed for the BaAl₄-type series of compounds. No superconductivity was encountered down to 2 K.