Tsunetomo Yamada

Effect of atom substitution on the low-temperature phase transitions in Cd- and Zn-based 1/1 cubic approximants

Abstract Effect of atom substitution on the low-temperature phase transitions in binary Cd6Ca and Zn6Sc 1/1 cubic approximants were investigated by means of electrical resistivity and X-ray diffraction. By adding thirdelements such as Yb, Cu and Fe to the binary approximants, it is concluded that Yb is substituted to the icosahedron site in both Cd6Ca and Zn6Sc, and Cu and Fe are substituted to the Zn sites in Zn6Sc. Upon the substitutions of the third elements, the temperature of the resistivity anomaly decreases without an exception and the anomaly disappears above a certain level of substitutions. Moreover, it is found that the chemical disorder introduced in the 2nd shell of the icosahedral cluster plays a crucial role in the suppression of the ordering of the Zn tetrahedron. On the other hand, the chemical disorder introduced in the 3rd icosahedron shell affects the transition temperature (Tc) rather weakly. Finally, the variation of Tc has been found to depend on the difference of the atomic radius between the host and the guest elements and the distortion of the outer shells is considered to be mainly responsible for the suppression of the ordering of the tetrahedra.

In-situ high-pressure X-ray diffraction on the Zn6Sc 1/1 periodic cubic approximant to a quasicrystal

Abstract The Zn6Sc 1/1 cubic approximant to a quasicrystal has been studied in-situ at high pressures by single-crystal X-ray diffraction. This phase can be described as a bcc packing of Tsai-type icosahedral clusters whose center is occupied by a disordered Zn4 tetrahedron. At ambient pressure the Zn6Sc undergoes a structural phase transition at 159 K to a monoclinic superstructure in which the Zn4 tetrahedra are orientationally ordered along the [1̄01] direction of the high-temperature bcc phase. In the pressure range up to 35 GPa, two new superstructures have been observed. The second phase corresponds to a four-fold pseudo cubic superstructure, i.e. a very large unit cell with a lattice parameter of about 5.5 nm. The resulting pressure-temperature phase diagram is different from that of Cd6Yb, which was reported by Watanuki et al. (2006).

Atomic structures of the Sc–Zn and R–Cd icosahedral quasicrystals

We present the refinement results of atomic structures of binary icosahedral (i) ScZn7.3, GdCd7.9, DyCd7.5 and TmCd7.3 quasicrystals (QCs) [1, 2] and the study of phason modes. Bragg peak intensiti ...

Intermediate-valence Yb-based quasicrystals and approximants

We are searching for intermediate-valence (IV) quasicrystals where IV ions are quasi-periodically arranged. Our previous study revealed that an icosahedral Au-Al-Yb quasicrystal forms an IV system [1]. X-ray absorption spectroscopy near the Yb L_3 edge indicated that the Yb ions in the quasicrystal assume a mean valence of 2.61, between a divalent state (4f^14, J = 0) and a trivalent one (4f^13, J = 7/2). Additionally we found that non-Fermi-liquid behaviour appears at low temperatures without doping, pressure, or field tuning. In this study we examined Au-M-Yb system, where M = Sn, Ge, and Ga. X-ray absorption spectroscopy measurements at SPring-8 (BL22XU) showed that each compounds forms IV system. The Yb valence values are respectively 2.27 for a Au-Sn-Yb 2/1 crystalline approximant, 2.24 for a Au-Ge-Yb 1/1 approximant, and 2.36 for a Au-Ga-Yb 1/1 approximant. By following the trend of Ybbased IV compounds, these valence values close to divalent suggest that the Yb 4f character in these compounds would be itinerant rather than localized one.