Properties of Cd-based binary quasicrystals and their 1/1 approximants

Electronic and structural properties of Cd-based binary quasicrystals (QCs) and their 1/1 approximants (APs) without chemical disorder are overviewed in terms of electronic states, electronic transport, and phase transitions. A pseudogap in the electronic density of states (DOS) as well as hybridization effect between sp and d states are discussed for both the QCs and their 1/1 APs in comparison with theoretical calculations. In contrast with the cases of ternary stable QCs, the resistivity of the Cd _5.7Yb QC exhibits a positive temperature coefficient at low temperatures, which is regarded as an indication of an intrinsic metallic nature of the QC. A unique structural phase transition has been observed at low temperatures (LT) for a series of 1/1 Cd ₆M APs, which is interpreted as an orientational transition of a tetrahedron residing at the center of the Tsai-type icosahedral cluster. In most of the cases, the LT superstructure is understood as an " antiferromagnetic" ordering of the tetrahedra along a <110> direction of the high-temperature bcc phase. The transition is classified as a non-diffusive order-disorder transition, which has not been observed in metallic alloys before.