Ternary Rare Earth (R) Transition Metal Aluminides R3T4Al12 (T = Ru and Os) with Gd3Ru4Al12 Type Structure

The isotypic intermetallic compounds R₃Ru₄Al₁₂ (R = Y, Pr, Nd, Sm, Gd—Tm) and R₃Os₄Al₁₂ (R = Y, Ce—Nd, Sm, Gd—Tm) were prepared by reaction of the elemental components in an arc‐melting furnace. Their crystal structure was determined from four‐circle X‐ray diffractometer data of Y₃Ru₄Al₁₂: P6₃/mmc, a = 877.7(1) pm, c = 952.3(1) pm, Z = 2, R = 0.028 for 361 structure factors and 28 variable parameters. It was also refined for Nd₃Os₄Al₁₂ (a = 889.2(1) pm, c = 960.3(1) pm, R = 0.021 for 425 F values and 30 variables) and Gd₃Os₄Al₁₂ (a = 884.7(1) pm, c = 955.3(2) pm, R = 0.020; 427 F values, 30 variables). The refinements of the occupancy parameters revealed mixed T/Al occupancy for some of the aluminum sites resulting in the compositions Y₃Ru_4.060(3)Al_11.940(3), Nd₃Os_4.43(1)Al_11.57(1), and Gd₃Os_4.44(1) Al_11.56(1), respectively. The structure is related to those found for Y₂Co₃Ga₉, Er₄Pt₉Al₂₄, CeOsGa₄, Ho₃Ru₄Ga₁₅, YbFe₂Al₁₀, TbRe₂Al₁₀, LuRe₂Al₁₀, and CaCr₂Al₁₀. Topologically all of these structures may be viewed as consisting of atomic layers, although chemical bonding within and between the layers is of similar character. Two kinds of layers can be distinguished in these structures. One kind contains all of the rare earth (occasionally also alkaline earth) and in addition aluminum or gallium atoms. The other kind of layers consists of the transition metal atoms and again aluminum or gallium atoms. These latter layers are hexa gonally close packed and slightly puckered. The three different structures of the disilicides TiSi₂, CrSi₂, and MoSi₂ also contain these layers; however, in the disilicides these layers are flat.