STRUCTURE OF GD3RU4AL12, A NEW MEMBER OF THE EUMG5.2 STRUCTURE FAMILY WITH 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 substitution variant of the Sc3Ni11Ge4 structure which is a partly disordered variant of the Sc3Ni11Si4 type, all three structures being ternary substitution derivatives of the EuMg5.2 or ErZn5 types. Contrary to the 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 layers of composition Gd3Al4 and puckered layers of composition Ru4Al8. Similar layers are found in cubic Hf6Ni7Al16, a ternary substitution variant of Th6Mn23, as well as in hexagonal Sr9Mg38 and BaLi4.