J.Q. Guo

Crystal and quasicrystal structures in Cd?Yb and Cd?Ca binary alloys

We show that the structures of cubic Cd6Yb and Cd6Ca crystals are closely related to those of recently found stable icosahedral Cd-Yb and Cd-Ca alloys. These structures are described as a periodic packing of rhombohedra having different atomic decoration in terms of three-dimensional Penrose tiling. A new type of icosahedral cluster composed of 66 atoms is found. The innermost shell of the cluster consisting of four Cd atoms breaks its entire icosahedral symmetry.

Stable Icosahedral Quasicrystals in the Cd–Mg–RE (RE = Rare Earth Element) Systems

A stable icosahedral phase has been found in a series of Cd65Mg20RE15 (RE=Y, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) alloys. These icosahedral alloys have quasilattice parameters from 0.5571 to 0.5799 nm. Electron diffraction study confirmed that the icosahedral quasicrystals have a primitive icosahedral structure.

Stable Cd-Mg-Yb and Cd-Mg-Ca icosahedral quasicrystals with wide composition ranges

Stable icosahedral quasicrystals with wide composition ranges have been found in the systems Cd-Mg-Yb and Cd-Mg-Ca. The icosahedral quasicrystals form in the following composition ranges: 25-85at.%Cd, 0-60at.%Mg and 10-20at.%Yb for the Cd-Mg-Yb system; 33-85at.%Cd, 0-52at.%Mg and 10-20at.%Ca for the Cd-Mg-Ca system. An electron diffraction study confirmed that the icosahedral quasicrystals have a primitive lattice. Its quasilattice constant is proportional to a Goldschmidt atom diameter of the quasicrystals weighted by a factor of 1.75 in the range 0.5681- 0.5799nm for the Cd-Mg-Yb system and 0.5697-0.5778nm for the Cd-Mg-Ca system. The closely similar atom sizes of Cd and Mg, the valence electron ratio e / a =2.0 and the large atom sizes of Yb and Ca cause the wide composition ranges of the Cd-Mg-Yb and Cd-Mg-Ca quasicrystals. The metallurgical behaviour of the quasicrystals is discussed.

Surface structure and structural transition of decagonal Al-Ni-Co quasicrystal

Abstract X-ray photoelectron diffraction (XPD) and reflection high-energy electron diffraction (RHEED) measurements have been performed to investigate the surface structure of the decagonal Al 72 Ni 12 Co 16 quasicrystal, which is known as a two dimensional quasicrystal. RHEED patterns for the surface parallel to the quasiperiodic planes are almost identical to the selected area electron diffraction (SAD) patterns taken along the twofold axes of the bulk sample. The XPD images show clear tenfold-symmetric patterns consistent with the long range orientational order of the decagonal quasicrystal. These facts suggest that the surface has the same quasiperiodic structure as in the bulk. The surface phase transition induced by Ar + ion bombardment is also investigated.

Production of single quasicrystals and their electrical resistivity in the Al-Pd-Re system

Single Al-Pd-Re icosahedral quasicrystals with a maximum diameter of 5mm have been grown by a slow cooling method on the basis of a partial phase diagram determined in the present study. Laue X-ray and electron diffraction verified the highly ordered structure of the single icosahedral quasicrystals. The electrical resistivity rho of the single quasicrystals was measured to be 2000- 4000muOmegacm at 300K and 3000-6000muOmegacm at 2K, revealing a negative temperature dependence with a rho4.2K/rho300K value smaller than 2.

Production of Single Decagonal Quasicrystal in Al-Co-Cu System

An Al-Co-Cu vertical phase diagram along the 65 at% Al isopleth was constructed with the aim of growing a single decagonal quasicrystal. The decagonal quasicrystal was formed at 1000?C via a peritectic reaction between a liquid and a cubic Al55(Co, Cu)45 phase. The decagonal quasicrystal crystallized as a primary phase for alloys with Co content less than 8.5 at% over a temperature range of 650 to 1000?C. On the basis of the phase diagram, a single decagonal quasicrystal with a composition of Al66Co17Cu17 was grown from a starting alloy of Al65Co7.5Cu27.5 by the Bridgman method.

A new stable icosahedral quasicrystal in the Cd-Mg-Dy system

A new stable icosahedral quasicrystal has been found in annealed Cd-Mg-Dy alloys. The composition of the icosahedral phase was determined to be approximately Cd66Mg21Dy13. Powder X-ray and electron diffraction patterns revealed that the phase has a primitive icosahedral lattice with a quasilattice parameter aR = 0.5634 nm. The electron diffraction study confirmed that the phase has a well ordered primitive icosahedral structure.

Magnetic properties of the icosahedral Cd-Mg-rare-earth quasicrystals

We report dc and ac magnetization of the newly discovered Cd-Mg-RE quasicrystals (RE = Gd, Tb, Dy, Ho, Er, Tm and Yb). For all the RE atoms except Yb, the temperature dependence of the dc magnetization obeys the Curie-Weiss law at higher temperatures, T>50 K. Estimated effective moments indicate that these atoms are in the trivalent states. In contrast, the Cd-Mg-Yb quasicrystal shows very small magnetization, suggesting that Yb is in the non-magnetic divalent state. At lower temperatures, spin-glass-like freezing was observed for the RE atoms except Tm and Yb. In particular, the freezing proceeds in two successive steps for RE = Gd, Tb, Dy and Ho, being quite different from canonical spin-glasses.

Nano-quasicrystalline phase formation in Mg-Cd-Yb alloys

Abstract The microstructure of Mg 60+ x Cd 25− x Yb 15 , x =0, 5, 10, Mg 50 Cd 30 Yb 20 and Mg 25 Cd 65 Yb 10 as-cast alloys has been examined by transmission electron microscopy. While the Mg 60 Cd 25 Yb 15 alloy showed only an icosahedral quasicrystalline phase, the magnesium rich alloys also showed a Zn 2 Mg type (Mg,Cd) 2 Yb Laves phase. The alloys showed nanosized particles of icosahedral phase. Eutectic microstructures involving the icosahedral phase were observed. A lamellar (50 nm) structure of icosahedral phase and Mg was observed, exhibiting a definite orientation relationship with the hexagonal axis along one of the twofold axes of the icosahedral phase. In the Cd-rich alloys the cubic Cd 6 Yb approximant phase occurred, along with the icosahedral phase and intermediate structures in nanocrystalline form.

Spin freezing in icosahedral Tb–Mg–Zn and Tb–Mg–Cd quasicrystals

The nature of spin freezing in geometrically frustrated icosahedral quasicrystals Tb–Mg–Zn and Tb–Mg–Cd was studied by thermoremanent dc magnetization (TRM) decay as a function of aging time and magnetic field. At low temperatures the magnetization exhibits typical broken-ergodicity phenomena, as characteristic of spin glasses (SGs). However, the observed linear dependence of the TRM on the magnetic field in the low-field regime is incompatible with the aging of a nonergodic system in an ultrametrically organized free energy of a SG, but compatible with a single-global-minimum free energy of a superparamagnet below the blocking temperature. The Tb–Mg–Zn(Cd) quasicrystals are, from this point of view, different from site-disordered SGs, but similar to geometrically frustrated pure (site-ordered) systems, like the kagome and pyrochlore antiferromagnets, which also exhibit a superparamagnetic component in the magnetization below the spin freezing temperature and clustering of spins. The Tb–Mg–Zn(Cd) quasicrystals show features associated with both the site-disordered SGs and the superparamagnets. This duality is not a specific feature of spins in a quasiperiodic structure, but is found quite commonly in nonrandom (site-ordered) geometrically frustrated magnetic systems.