Order–disorder transition in cubic Cd6Yb and Cd6Ca

Specific heat, X-ray diffraction and electron diffraction experiments at low temperatures have clearly shown that the Cd6Yb and Cd6Ca crystals, which are 1/1 cubic approximants at room temperature, undergo an order–disorder transition at 110 and 100 K, respectively. The transition is reasonably attributed to the orientational ordering of 4Cd tetrahedra located at the center of the icosahedral clusters; above 110 K (100 K) the 4Cd tetrahedra are completely disordered resulting in a bcc structure, whereas below 110 K (100 K) they are ordered in a non-cubic structure of a doubled unit cell.