Group theoretical treatment of the low-temperature phase transition of the Cd6Ca 1 1-cubic approximant

An antiparallel orientational transition is reported for an intermetallic compound, i.e., Cd6Ca crystal, which is a 1 1-1 1-1 1 crystalline approximant to the icosahedral quasicrystal Cd5.7Ca. A group theoretical analysis based on the Landau theory predicts that the space group of the low-temperature phase is either C2 c or C2 m, in good agreement with the observations. Accordingly, two types of orientational orderings of Cd4 tetrahedra, which are located in the center of icosahedral clusters, may occur below 100K: In both cases, the Cd4 tetrahedra are orientationally ordered in an antiparallel fashion along the [110] direction of the high temperature body-centered-cubic phase. Such a transition in a metal is reminiscent of orientational transitions in molecular solids.