Icosahedral quasicrystal and 1/1 cubic approximant in Au-Al-Yb alloys

A P-type icosahedral quasicrystal is formed in Au-Al-Yb alloy of which 6-dimensional lattice parameter a6D = 7.448 {\AA}. The composition of the quasicrystal was analyzed to be Au51Al34Yb15. This quasicrystal is formed in as-cast alloys, and is regarded as metastable because of decomposition into other crystalline phases by annealing at 700 \degree C. Among Tsai-type quasicrystals, this quasicrystal is situated just between Zn-Sc group with smaller a6D and larger Cd-Yb group. Intermediate valence of Yb recently observed in this quasicrystal may be due to this unique situation, namely smaller major component Au-Al than in Cd-Yb. The predominant phase in the annealed specimen is a 1/1 cubic approximant with lattice parameter a = 14.500 {\AA} belonging to the space group Im-3 . This phase is stable at the composition Au51Al35Yb14 at 700 \degree C. Rietveld structural analysis indicated that the crystal structure is understood as periodic arrangement of Tsai-type clusters each including four Au-Al atoms at their centers. Chemical ordering of Au and Al is characteristics of this approximant.Comment: 20 pages including 1 table and 7 figure

Neutron scattering study on spin correlations and fluctuations in the transition-metal-based magnetic quasicrystal Zn-Fe-Sc

Spin correlations and fluctuations in the 3d-transition-metal-based icosahedral quasicrystal Zn-Fe-Sc have been investigated by neutron scattering using polycrystalline samples. Magnetic diffuse scattering has been observed in the elastic experiment at low temperatures, indicating development of static short-range-spin correlations. In addition, the inelastic scattering experiment detects a $Q$-independent quasielastic signal ascribed to single-site relaxational spin fluctuations. Above the macroscopic freezing temperature $T_{\rm f} \simeq 7$ K, the spin relaxation rate shows Arrhenius-type behavior, indicating thermally activated relaxation process. In contrast, the relaxation rate remains finite even at the lowest temperature, suggesting a certain quantum origin for the spin fluctuations below $T_{\rm f}$.Comment: To be published in Phys. Rev.

Icosahedral quasicrystals in Zn-T-Sc (T=Mn, Fe, Co, Ni) alloys

Starting from the Zn17Sc3 cubic approximant, new quasicrystal alloys were searched by replacement of Zn with transition elements, T. In the cases of T=Mn, Fe, Co and Ni, new icosahedral quasicrystals are formed in as-cast alloys as major phases at the alloy compositions of Zn75T10Sc15. All these quasicrystals belong to a primitive type, and have 6-dimensional lattice parameters, a6D, ranging from 7.044 to 7.107 A. They have the valence electron concentrations, e/a, ranging from 2.01 to 2.14, and almost the same ratios between the edge-length of the Penrose tile, aR, and the averaged atomic diameter d: aR/d~1.75. Moreover other Zn- and Cd-based quasicrystals including the same type of atomic cluster, Tsai-type cluster, also have the same values of e/a~2.1 and aR/d~1.75. The equality in e/a indicates that the Hume-Rothery mechanism plays an important role for the formation of these quasicrystals.Comment: 9 pages, 2 tables, 3 figure