Two series of rare earth metal-rich ternary aluminium transition metallides – RE 6Co2Al (RE=Sc, Y, Nd, Sm, Gd–Tm, Lu) and RE 6Ni2.25Al0.75 (RE=Y, Gd–Tm, Lu)

The rare earth metal-rich cobalt and nickel aluminium compounds with the general compositions RE 6Co2Al (RE=Sc, Y, Nd, Sm, Gd–Tm, Lu) and RE 6Ni2.25Al0.75 (RE=Y, Gd–Tm, Lu) have been synthesised from the elements by arc-melting, followed by annealing. Single-crystal X-ray diffraction experiments on Y6Co2.02(1)Al0.98(1) (Ho6Co2Ga type; Immm; a=944.1(2), b=952.4(2), c=999.0(2) pm; wR2=0.0452, 1123 F 2 values, 35 variables) and Y6Ni2.26(1)Al0.74(1) (Ho6Co2Ga type; Immm; a=938.30(5), b=959.45(5), c=996.05(6) pm; wR2=0.0499, 1131 F 2 values, 35 variables) revealed that the compounds form solid solutions according to the general formula RE 6(Co/Ni)2+ x Al1−x with different homogeneity ranges. The compounds of the Ni series can be obtained in X-ray pure form only with the nominal composition RE 6Ni2.25Al0.75. A significant increase of the U 22 component of the anisotropic displacement parameters of the Co/Ni2 atoms (4g site) was observed that requires a description of the structure with a split-position model at RT. Further investigations by low temperature (90 K) single-crystal X-ray diffraction experiments of Y6Co2.02(1)Al0.98(1) showed a significant decrease of U 22. Magnetic measurements were conducted on the X-ray pure members of the RE 6Co2Al (RE=Y, Dy–Tm, Lu) series. Antiferromagnetic ordering was observed for the members with unpaired f electrons with Néel temperatures up to T N=48.0(1) K and two spin reorientations for Dy6Co2Al.