On the crystal structure of the quasicrystalline approximant YbCd6 at low temperature

The body-centered cubic (space group Im3¯, No. 204) crystal structure of the quasicrystalline approximant YbCd6 has been studied by means of single-crystal X-ray diffraction at low temperature. These studies suggest that below 120 K, the disorder in the innermost Cd-shell in YbCd6 is best modeled with a 1/3 occupied cube-octahedron representing Cd4-tetrahedron in six possible orientations. The disordered cadmium atoms occupy the Wyckoff position 24g. Above this temperature, the disorder of the Cd4-tetrahedra inside the Cd20-dodecahedra in this “onion”-type structure becomes progressively larger and the Cd site can be further split as to occupy a general position with a Wyckoff letter 48f. These results are in agreement with the disorder model suggested previously from structure refinements of YbCd6 at room temperature.