Two series of rare earth metal-rich ternary aluminium transition metallides. RE₆Co₂Al (RE=Sc, Y, Nd, Sm, Gd-Tm, Lu) and RE₆Ni_2.25Al_0.75 (RE=Y, Gd-Tm, Lu)

The rare earth metal-rich cobalt and nickel aluminium compounds with the general compositions RE₆Co₂Al (RE=Sc, Y, Nd, Sm, Gd-Tm, Lu) and RE₆Ni_2.25Al_0.75 (RE=Y, Gd-Tm, Lu) have been synthesised from the elements by arc-melting, followed by annealing. Single-crystal X-ray diffraction experiments on Y₆Co_2.02(1)Al_0.98(1) (Ho₆Co₂Ga type; Immm; a=944.1(2), b=952.4(2), c=999.0(2) pm; wR2=0.0452, 1123 F² values, 35 variables) and Y₆Ni_2.26(1)Al_0.74(1) (Ho₆Co₂Ga type; Immm; a=938.30(5), b=959.45(5), c=996.05(6) pm; wR2=0.0499, 1131 F² values, 35 variables) revealed that the compounds form solid solutions according to the general formula RE₆(Co/Ni)_2+xAl_1-x with different homogeneity ranges. The compounds of the Ni series can be obtained in X-ray pure form only with the nominal composition RE₆Ni_2.25Al_0.75. A significant increase of the U₂₂ 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 Y₆Co_2.02(1)Al_0.98(1) showed a significant decrease of U₂₂. Magnetic measurements were conducted on the X-ray pure members of the RE₆Co₂Al (RE=Y, Dy-Tm, Lu) series. Antiferromagnetic ordering was observed for the members with unpaired f electrons with Neel temperatures up to T_N=48.0(1) K and two spin reorientations for Dy₆Co₂Al.