Structural chemistry and magnetic behavior of ternary gallides REAu xGa 4− x, RE = La, Ce, Pr, Nd, and Sm

Abstract

Two new series of ternary gallides with the chemical formulas (La, Ce, Pr, Nd, Sm)Au x Ga 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 x Ga 4− x series of compounds was found to be isotypic and crystallize with BaAl 4 type of structure; the homogeneity range at 600°C of each of the REAu x Ga 4− x phases was established, revealing remarkable deviations from Vegard's rule. At 600°C the REAu x Ga 4− x phases of the BaAl 4 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 (CaBe 2Ge 2 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 2 b sites. 197Au Mössbauer spectroscopy confirmed the occupational mode of the gold atoms in CeAu 1.5Ga 2.5. For the BaAl 4-type phases, X-ray powder and Mössbauer data revealed preferential occupation of the 4 e sites of I4 mmm by gold atoms; practically no Au was observed on the 4 d 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 x Ga 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 x Ga 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 BaAl 4-type series of compounds. No superconductivity was encountered down to 2 K.