A Low-Temperature Order-Disorder Transition in a Cubic Cd6Yb Crystalline Approximant

Electrical resistivity. specific heat and electron diffraction experiments at low temperatures, unambiguously reveal that the cubic Cd6Yb crystal. which is a 1/1 cubic approximant to the recently discovered icosahedral Cd-5 Yb-7, undergoes a reversible order-disorder transition at about I 10 K. The transition is most probably due to the orientational ordering of tetrahedral 4Cd clusters sitting at the center of dodecahedral cages: above 110 K the central 4Cd cluster is disordered over two orientations forming a bcc structure (Im (3) over bar), whereas below 110 K it is ordered in mixed B2(Pn (3) over bar) and B32(Fd (3) over barm) structures.