Universal low-temperature phase transition in Zn- and Cd-based crystalline approximants

Electron-diffraction studies have shown that a cubic crystal of Zn ₆Sc (Im 3̄, a=1.383 11 nm), which is a 1/1-1/1-1/1 crystalline approximant to the icosahedral quasicrystal, undergoes a phase transition at a low temperature above 80 K. The transition accompanies breakdown of the cubic symmetry as well as a change of the periodicity in the [110] direction, resulting in a 2ax2axa-type superlattice at low temperatures. This finding has led us to the conclusion that Zn tetrahedra, which were recently found in the center of the icosahedral cluster, exhibit orientational ordering at a low temperature, resulting in the symmetry breaking of the lattice. Furthermore, such a low-temperature phase transition is found to be a universal phenomenon of the Cd- and Zn-based 1/1-cubic approximants, verifying that the cubic approximants (Im 3̄) are not in a ground state but are entropically stabilized.