The crystal structure of Gd 5Si 4

Abstract

Gd 5Si 4 is orthorhombic, Pnma, lattice parameters, a = 7.4738(8), b = 14.724(1), c = 7.7362(7), Z = 4, at 25 °C. The structure has been determined from three-dimensional X-ray data collected by counter methods. Least-squares refinement has led to a final conventional R value (on F) of 0.061 for 917 observed reflections. The structure can be described in terms of three different layers perpendicular to the b axis. The first layer contains Si atoms only, the second only Gd atoms, and the third both Gd and Si atoms. The structure is similar to that of Sm 5Ge 4, but there are significant differences in the coordination number around several of the atoms. Two Gd atoms have 10 other rare-earth atoms as near neighbors as compared to 9 in the structure of Sm 5Ge 4. An additional 6 Si atoms form a very distorted octahedron around these two Gd atoms giving rise to a coordination number of 16. The third crystallographically independent Gd atom has a 14 fold coordination consisting of 8 Gd and 6 Si. This latter arrangement exists also for one of the rare-earth atoms in the structure of Sm 5Ge 4. The coordination number of one of the Si atoms has increased to 8 as compared to 7 in Sm 5Ge 4. The coordination of the other two crystallographically independent Si atoms remains at 9 as in the germanide compound.