Ternary Aluminides with the Ideal Composition A2Pt6Al15 (A = Y,Gd—Tm,Zr)

Ternary rare earth platinum aluminides were prepared by arc‐melting of the elemental components followed by annealing in a high‐frequency furnace. Their crystal structure was determined for the yttrium compound from four‐circle X‐ray diffractometer data. It has hexagonal symmetry with a = 428.1(1) pm, c = 1638.3(3) pm, space group P6₃/mmc, and was refined to a conventional residual of R = 0.018 for 325 F values and 19 variable parameters. Of the five crystallographic positions, the yttrium position and one of the three aluminum positions show partial occupancies corresponding to the composition Y_1.357(3)Pt₄Al_9.99(2) with the Pearson symbol hP20 — 4.65. These partially occupied sites are that close to each other that at best only one can be fully occupied. A model for an ordered distribution of occupied and unoccupied Y and Al sites requires a √3 larger a axis with the Pearson symbol hP20 — 4.67 for the subcell, very close to the experimental result. Corresponding superstructure reflections could be observed on an image‐plate single‐crystal diffractometer only in the form of diffuse streaks. The compound has the ideal composition Y₂Pt₆Al₁₅ with Z = 2 for the superstructure. This corresponds to the formula Y_1.33Pt₄Al₁₀ with Z = 1 for the subcell. The compounds A_1.33Pt₄Al₁₀ with A = Gd, Tb, Dy, Ho, Er, Tm were found to be isotypic with that of the yttrium compound. This structure is closely related to or isotypic with, respectively, those of Yb₂Fe₄Si₉, Sc_1.2Fe₄Si_9.8, Ce_1.2Pt₄Ga_9.8, Ce₂Pt₆Ga₁₅, Tb_0.67Ni₂Ga_5—xSi_x, RE_0.67Ni₂Ga_5—xGe_x> (with RE = Y, Sm, Ho), and Gd_0.67Pt₂Al₅, reported in earlier investigations. The new compound Zr_1.00(1)Pt₄Al_10.22(3) has nearly the same hexagonal structure with a = 426.1(1) pm and c = 1622.8(3) pm. It was refined from four‐circle diffractometer data to a residual of R = 0.021 for 288 structure factors and 19 variable parameters.