Shiro Kashimoto

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

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

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

A P-type icosahedral quasicrystal is formed in an Au–Al–Yb alloy with a six-dimensional lattice parameter a 6D = 7.448 Å. The composition of the quasicrystal was analyzed to be Au51Al34Yb15. The quasicrystal is formed in as-cast alloys and is regarded as metastable with decomposition to other crystalline phases during annealing at 700°C. Among Tsai-type quasicrystals, this quasicrystal is situated just between the Zn–Sc group with a smaller a 6D and the larger Cd–Yb group. The intermediate valence of Yb recently observed in this quasicrystal may be due to this unique situation, namely a smaller major component in Au-Al-Yb than in Cd–Yb. The predominant phase in the annealed specimen is a 1/1 cubic approximant with lattice parameter a = 14.500 Å belonging to the space group Im . This phase is stable at the composition Au51Al35Yb14 at 700°C. Rietveld structural analysis indicated that the crystal structure is a periodic arrangement of Tsai-type clusters with four Au–Al atoms at their centers. Chemical ordering of Au and Al is characteristics of this approximant.

Low-temperature phase of the Zn–Sc approximant

The low-temperature transition in the Zn–Sc 1/1 cubic approximant, reported by Tamura et al . [Phys. Rev. B 71 0902203 (2005)], has been confirmed by the following experimental evidence: (1) anomaly in temperature dependence of electrical resistivity at approximately 153 K, and (2) appearance of weak superlattice reflections both in electron and powder X-ray diffraction patterns. The low-temperature phase is monoclinic, belonging to the space group C2/c, with lattice parameters a = 1.947(5) nm, b = 1.379(3) nm, c = 1.955(5) nm and β = 89.931(3)°. The unit cell includes four unique Tsai-type clusters consisting of almost regular tetrahedron of Zn surrounded by triple shells. The first dodecahedral shell of Zn is highly distorted so as to avoid unrealistic short atomic distance to the Zn tetrahedron. Direct evidence of an orientational ordering of the clusters has been obtained by Reitveld analysis of the low-temperature phase. The occurrence of the transition is sensitive on alloy composition as well as heat treatment.

A New Series of Icosahedral Quasicrystals in Zn-M-Sc (M = Ag, Au, Pd, Pt) Alloys

A new series of icosahedral quasicrystals has been found in Zn75M10Sc15 (M = Ag, Au, Pd and Pt) as-cast alloys. Electron and powder X-ray diffraction analyses indicated that these quasicrystals are of the P-type (primitive type) and have six-dimensional lattice parameters a6D=0.7112–0.7151 nm. The quasicrystals are formed as main phases in these alloys. In the Zn80Ag5Sc15 alloy annealed at 981 K for 24 h, a Zn85Sc15-type cubic phase is formed exclusively, which has the lattice parameter a=1.3867 nm. The cubic phase is interpreted as a 1/1 approximant of Zn75Ag10Sc15. This observation, as well as intensity similarity in the X-ray diffraction patterns, suggests that Zn75M10Sc15 quasicrystals belong to the same group as the Zn80Mg5Sc15 quasicrystal from the viewpoint of a local atomic configuration.

Valence Change Driven by Constituent Element Substitution in the Mixed-Valence Quasicrystal and Approximant Au–Al–Yb

Quantum criticality has been considered to be specific to crystalline materials such as heavy fermions. Very recently, however, the Tsai-type quasicrystal Au51Al34Yb15 has been reported to show unusual quantum critical behavior. To obtain a deeper understanding of this new material, we have searched for other Tsai-type cluster materials. Here, we report that the metal alloys Au44Ga41Yb15 and Ag47Ga38Yb15 are members of the 1/1 approximant to the Tsai-type quasicrystal and that both possess no localized magnetic moment. We suggest that the Au-Al-Yb system is located near the border of the divalent and trivalent states of the Yb ion; we also discuss a possible origin of the disappearance of magnetism, associated with the valence change, by the substitution of the constituent elements.

Systematic study of magnetic properties in Zn-based Tsai-type icosahedral quasicrystals and their approximant

We have investigated the magnetic properties of Zn-based Tsai-type icosahedral quasicrystals (i-QCs) and their approximants by measuring the field-and temperature-dependence of magnetization. The quasicrystals are formed by an addition of the third element M (M = Mg, Mn, Fe, Co, Ni, Cu, Pd, Ag, Pt or Au) to a Zn17Sc3 1/1 cubic approximant crystal. The remarkable feature of the Zn–M–Sc series is a variety of the additional M elements ranging from light metal Mg and 3d transition metals to noble metals. The experimental results obviously show that the i-QCs containing 3d transition metals (M = Mn, Fe, Co or Ni) have the local magnetic moment formation. We also confirmed appearance of Curie-Weiss paramagnetism for a Zn82Fe3Sc15 1/1 cubic approximant crystal. The present results indicate that the divalent Zn of major component in the present i-QCs and approximants may be favourable for the moment formation of iron group elements in contrast with the case of the trivalent Al-based i-QCs such as Al–Cu–Fe.

Formation and magnetic properties of p-type icosahedral quasicrystals in Zn–Fe–Sc–L (L = Ho, Er, Tm) alloys

Icosahedral quasicrystals containing heavy lanthanoids L, namely Ho, Er or Tm, have been discovered in the Zn77Fe7Sc16– x L x alloy systems. The solubility limit of the lanthanoid is approximately half of the Sc atoms. All these quasicrystals belong to p-type and are stable at approximately 973 K. The temperature dependence of the magnetic susceptibility of the Zn77Fe7Sc8Tm8 quasicrystal follows the Curie–Weiss law and exhibits spin-glass-like behaviour with a freezing temperature of 9 K. The Curie constants obtained by Curie–Weiss fit are larger than those calculated by the Lande g-factor of free Tm3+ ions based on Hunds rule, assuming all magnetization is caused by Tm atoms only. It is deduced that both Tm and Fe atoms exhibit magnetic moments. This is the first example of a stable icosahedral quasicrystal containing two types of magnetic atoms: Fe and the lanthanoids.

Magnetic Property of Stable Icosahedral Quasicrystal in Zn-Fe-Sc Alloy

We report the observation on the magnetic property of the stable Zn77Fe7Sc16 icosahedral quasicrystal discovered recently. As a result of measurements of magnetization, it became clear that the new Zn-based icosahedral quasicrystal has large local magnetic moments and exhibits spin glass behavior. The magnetization curves show Curie-like paramagnetism, where the magnetization M is 4.45 cgsemu/g at 5 K under 70 kOe. The temperature dependence of the magnetic susceptibility obeys the Curie-Weiss law between 76 and 300 K. On the assumption that all of the Fe atoms have the same magnitude of local magnetic moment, the effective number of Bohr magneton p of each Fe atom was estimated at 5.30 on average. Moreover, in the low-temperature region, the spin glasslike cusp of the magnetic susceptibility was observed with a freezing temperature Tf = 7.2 K.

Magnetic property of a Zn-Mg-Sc icosahedral quasicrystal

We have investigated the magnetism of a new Zn 80 Mg 5 Sc 15 P-type icosahedral quasicrystal and a Zn 85.5 Sc 14.5 cubic phase, which is interpreted to be a 1/1 approximant of the new icosahedral quasicrystal, in the temperature region between 2 and 700 K. The magnetic susceptibility of the Zn 80 Mg 5 Sc 15 icosahedral quasicrystal shows an increase from -1.3×10 -7 to -0.8×10 -7 cgsemu/g with a rise in temperature from 90 to 700 K. The Zn 85.5 Sc 14.5 cubic phase shows also a rise from -1.8×10 -7 to -1.0×10 -7 cgsemu/g between 120 and 700 K. The increase in the susceptibility with a rise in temperature is accounted for by a temperature dependence of the Pauli paramagnetism. The experimental results strongly suggest a pseudogap in the electronic density of states at Fermi energy for the Zn 80 Mg 5 Sc 15 quasicrystal and the Zn 85.5 Sc 14.5 approximant, which has been observed in various stable icosahedral quasicrystals.

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

Spin correlations and fluctuations in the