Crystal Chemistry and Magnetic Properties of the Ternary Compounds RE₃Ag_xGa_11-x (RE = Y, Gd, Tb, Dy, Ho, Er, Tm, and Yb)

Ternary samples with the composition RE3 AgxGa11-x, RE = Y, Gd, Tb, Dy, Ho, Er, Tm, and Yb, have been synthesized from the elements by arc- or HF-melting and annealing at 600°C. In all cases isotypy with the La3Al11-type has been confirmed from X-ray powder data. The homogeneity region of the Yb3AgxGa11-x phase has been established by X-ray powder diffraction analysis and ranges at 600°C from x = 2.6 to x = 3.8. Unit cell parameters in this region reveal a remarkable positive deviation from Vegard's rule corresponding with a partial composition-dependent atomic ordering in the various sites of the La3Al11-type structure. The occupation mode in the Yb-containing alloys has been studied in detail by full profile X-ray powder analysis for compositions x = 2.55 and 3.65 and by X-ray single crystal counter data refinement for the alloy with x = 3.0, confirming in all cases full consistency with the La3Al11-type structure with the space group Immm. Throughout the homogeneous range Ag was observed to preferentially occupy the (2c) sites. The relative increase of the Ag occupancy in (2c) with increasing content of silver, however, decreases and for compositions x > 3.0, Ag was found to enter the (81) positions. In correspondence with magnetic susceptibility measurements the peculiar lattice parameter variation observed is due to this complicated mode of Ga/Ag-substitution rather than to a valence change on the ytterbium atom. Magnetic susceptibilities were determined over a temperature range from 4 to 550 K. Above LNT, magnetic behavior of Gd (x = 2.8), Tb, Dy, Ho, Er, and Tm compounds (x = 3.8) corresponds to the paramagnetism of ideal tripositive rare earth elements. Below T = 25 K antiferromagnetic ordering of the RE-moments is encountered for the Yb3AgxG11-x alloys. The Yb-atom was found to be in a nonmagnetic Yb2+ ground state throughout the entire homogeneous range. The susceptibilities are practically temperature independent paramagnetic down to 20 K. Y3Ag3Ga8 is a diamagnet