Low-temperature transmission electron microscopy (TEM) studies on the Cd6M compounds reveal the predominant occurrence of orientational ordering of a tetrahedron inside a dodecahedral cage.

If a low-symmetry object is inside a high-symmetry cage in a solid, what happens? At elevated temperatures such low-symmetry objects will be disordered to gain entropy, if the melting point of the solid is high enough. On the other hand, at low temperatures they will be ordered to reduce energy, provided that their rotation is kinetically allowed. Hence, a new type of order–disorder transition intrinsic to high-symmetry clusters may occur. In J. Phys.: Condens. Matter 25 235403, we investigated all the available Cd6M (M = Ca, Y, Sr, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) compounds, and report that the same type of phase transition prevails in most of them.

The orientational type of phase transition was first recognized by one of the authors (Tamura et al 2002) in Cd6Yb. In the Cd6M compounds, a tetrahedron of four Cd atoms resides inside a dodecahedron of 20 Cd atoms, which is located at each bcc lattice point. If we assume a regular dodecahedron, for an arbitrary orientation of a tetrahedron, there are a huge number of equivalent orientations, i.e. 120! Of course, the dodecahedron is distorted because of the crystal field in the cubic compounds, but still the tetrahedron is orientationally disordered over a number of orientations at room temperature.

A couple of years ago, it was almost concluded that such a phase transition does not occur for smaller rare-earth elements, such as Ho, Er, Tm and Lu based on our low-temperature TEM studies. It was only by chance that Cd6Ho was found to exhibit a phase transition at a low temperature last year. This happened during a synchrotron x-ray magnetic scattering experiment with Alan Goldman's group from Iowa State University, where a Cd6Ho sample that had been stored at room temperature for more than a year was found to exhibit superlattice reflections below 178 K.

We restarted investigations on the possible occurrence of a phase transition in the other Cd6M and successfully observed ordering of the tetrahedron in Cd6M (M = Er, Tm) for samples stored for 2.4 years in our laboratory. Only for Lu, no transition has been observed to date, even in samples stored for 10 years! We believe that Cd6Lu will also exhibit a phase transition in the future but we are not sure how many years it will take. A similar phenomenon is expected in other solids composed of highly symmetric clusters which contain a low-symmetry object.