New quaternary R2CoAl4Si2 compounds

Eight new R₂CoAl₄Si₂ silicides (R = Y, Gd-Yb) were synthesized by arc melting, and their crystal structures were studied by X-ray powder diffraction. The compounds were found to adopt the same tetragonal structure type Tb₂NiAl₄Ge₂ (Pearson symbol tI18, space group I4/mmm) and the cell parameters refined to: a = 4.09319 (6), c = 19.3431 (4) Å (R = Y), a = 4.1109 (3), c = 19.425 (2) Å (R = Gd), a = 4.09788 (8), c = 19.3616 (5) Å (R = Tb), a = 4.0850 (1), c = 19.2967 (8) Å (R = Dy), a = 4.07802 (2), c = 19.2575 (2) Å (R = Ho), a = 4.06871 (8), c = 19.2082 (5) Å (R = Er), a = 4.05836 (4), c = 19.1599 (4) Å (R = Tm), and a = 4.06627 (6), c = 4.06627 (6) Å (R = Yb). Isotypic compounds with R = Sm, Lu were not observed under the experimental conditions.

The structure type Tb₂NiAl₄Ge₂ is a quaternary variant of the prototype Yb₃S₂F₄ ≡ (NiGe₂)Tb₂Al₄. The structure of the R₂CoAl₄Si₂ compounds can be described as a packing of monocapped square antiprisms [SiAl₄R₅] and cubes [CoAl₈]. It can also be considered as formed by two types of layer containing cubes, stacked along the crystallographic direction [001]. One kind of layer is built of cubes of composition Al₈, with every second cube centered by a Co atom, while the other kind of layer consists of slightly deformed empty cubes of composition R₄Si₄.