Crystal Structure of the Compound Sc_(1.33)Pd_3Al_8 with Layers of R Atoms and Al_3 Triangles

摘要

The crystal structure of the new ternary aluminide Sc_(1.33)Pd_3Al_8 was refined by the Rietveld method from X-ray powder diffraction data. It crystallizes with a Gd_(1.33)Pt_3Al_8-type structure: hR51-14.00, R-3m, a = 4.29142(4), c = 38.1638(4) A, R_B = 0.0344. The main feature of the structure is the statistical distribution of Sc atoms and Al_3 triangles within atomic layers of composition Sc_2Al_3 (Sc_(0.67)Al within the translation unit here), which is likely to correspond to stacking disorder of ordered layers. During the final cycles of the refinement, the occupancies of the corresponding sites were fixed at occ. = 2/3 for Sc in Wyckoff position 6c and occ. = 1/3 for Al in 18h. The unit cell of Sc_(1.33)Pd_3Al_8 contains six Sc_(0.67)Al layers, nine Pd and eighteen Al atom layers along the crystallographic direction [001]. Together with the structure types Tb_(0.67)PdAl_3, Y_2Co_3Ga_9, Sc_(0.67)Fe_2Si_5, Er_4Pt_9Al_(24), Yb_(0.67)Ni_2Al_6, and ErNi_3Al_9, the structure type Gd_(1.33)Pt_3Al_8 forms a family of intergrowth structures built up of three kinds of similar monoatomic layer.