STRUCTURE OF GD3RU4AL12, A NEW MEMBER OF THE EUMG5.2 STRUCTURE FAMILYWITH MINORITY-ATOM CLUSTERS

Gd3Ru4+xAl12-x, [x = 0.13 (2)), M(r) = 1209.44, hexagonal, hP38, P6(3) /mmc - kh2gf ba (No. 194), a = 8.8142 (6), c = 9.5692 (9) angstrom, V = 643.8 (1) angstrom3, Z = 2, D(x) = 6.239 Mg m-3, lambda(Mo Kalpha) = 0.71073 angstrom, mu = 20.330 mm-1, F(000) = 1056 (1), T = 293 K, wR = 0.023 for 338 contributing unique reflections. Gd3Ru4Al12 is a subst itution variant of the Sc3Ni11Ge4 structure which is a partly disorder ed variant of the Sc3Ni11Si4 type, all three structures being ternary substitution derivatives of the EuMg5.2 or ErZn5 types. Contrary to th e related structure types, in Gd3Ru4Al12 both Wyckoff sites 2(a) and 2 (b) have full occupation, i.e. an ordered arrangement of alternate Ru and Al atoms is observed along the c axis. The structure of Gd3Ru4Al12 is built up of two kinds of layers perpendicular to [001]: planar lay ers of composition Gd3Al4 and puckered layers of composition Ru4Al8. S imilar layers are found in cubic Hf6Ni7Al16, a ternary substitution va riant of Th6Mn23, as well as in hexagonal Sr9Mg38 and BaLi4.