Tsai-type Quasicrystals Research Articles

Growth of pentacene molecules on Tsai-type quasicrystals and related crystal surfaces

We present a study of the adsorption of pentacene (Pn) molecules on the high symmetry (fivefold, threefold, and twofold) surfaces of the icosahedral (i) Ag–In–Yb quasicrystal. We also compare the results with adsorption of Pn on a surface of a periodic crystal related to this quasicrystal, the (111) surface of the Au–Al–Tb 1/1 approximant. Scanning tunneling microscopy reveals that Pn molecules on the quasicrystal surfaces are aligned along the high symmetry directions of the substrates and selectively adsorb on Yb atoms and thus exhibit quasicrystalline order. Pn molecules on the Au–Al–Tb approximant surface also preferably adopt Tb sites. The behavior of selective adsorption can be understood in terms of the geometry and electronic properties of the adsorbate and substrate. The Yb–Yb (Tb–Tb) separations are comparable to the C–C or H–H distances in a Pn molecule. Pn is an electron donor, whereas the unoccupied electronic states of the substrate are dominated by the rare earth atoms, suggesting that there is an electronic transfer between the Pn molecules and Yb (Tb) atoms.

Atomic-Scale Tuning of Tsai-Type Clusters in RE-Au-Si Systems (RE = Gd, Tb, Ho).

Tsai-type quasicrystals and approximants are distinguished by a cluster unit made up of four concentric polyhedral shells that surround a tetrahedron at the center. Here we show that for Tsai-type 1/1 approximants in the RE–Au–Si systems (RE = Gd, Tb, Ho) the central tetrahedron of the Tsai clusters can be systematically replaced by a single RE atom. The modified cluster is herein termed a “pseudo-Tsai cluster” and represents, in contrast to the conventional Tsai cluster, a structural motif without internal symmetry breaking. For each system, single-phase samples of both pseudo-Tsai and Tsai-type 1/1 approximants were independently prepared as millimeter-sized, faceted, single crystals using the self-flux synthesis method. The full replacement of tetrahedral moieties by RE atoms in the pseudo-Tsai 1/1 approximants was ascertained by a combination of single-crystal and powder diffraction studies, as well as energy dispersive X-ray spectroscopy (EDX) analyses with a scanning electron microscope (SEM). Differential scanning calorimetry (DSC) studies revealed distinctly higher decomposition temperatures, by 5–35 K, for the pseudo-Tsai phases. Furthermore, the magnetic properties of pseudo-Tsai phases are profoundly and consistently different from the Tsai counterparts. The onset temperatures of magnetic ordering (Tmag) are lowered in the pseudo-Tsai phases by ∼30% from 24 to 17 K, 11.5 to 8 K, and 5 to 3.5 K in the Gd–Au–Si, Tb–Au–Si, and Ho–Au–Si systems, respectively. In addition, the Tb–Au–Si and Ho–Au–Si systems exhibit some qualitative changes in their magnetic ordering, indicating decisive changes in the magnetic state/structure by a moment-bearing atom at the cluster center.

Analyzing by the Two-Level System and Conduction Mechanism of Icosahedral Quasicrystals CdYb and CdMgYb

We have presented one of possible mechanisms for anomalous transport properties in Tsai-type quasicrystals CdYb and CdMgYb. The tunneling process between the Tsai-type clusters has been argued in dilute bound approximation of dynamics of the two-level system.

Studies on icosahedral Ag–In–Yb: a prototype for Tsai-type quasicrystals

Tsai-type icosahedral quasicrystals (i-QCs) are present in numerous alloy systems. Among them, the icosahedral Ag–In–Yb quasicrystal (i-Ag–In–Yb) is regarded as a prototype for studying various features of QCs, such as crystalline structure and physical properties, due to its good average structural order, good stability both in air and in ultra-high vacuum, amenability to growth of large single grains, and lack of elements with high vapor pressure or toxicity. Here we present a review on recent studies of i-Ag–In–Yb, including studies on bulk structure, crystal growth, physical properties, surface structure and thin film growth on quasicrystalline surface. We believe the results will encourage studies on other QCs.

Synthesis and thermal stability of Au-based quasicrystals and its approximants

Various Tsai-type quasicrystals and crystalline approximants have been found in a variety of systems. These compounds are made of Tsai-type icosahedral clusters. Recently, the existence of long-range magnetic orders was reported in Cd6R and Au-SM-R approximants[1,2]. Therefore, it is of interest to investigate the magnetic properties of new alloys systems which are also composed of Tsai-type clusters. In the present work, in order to search new Tsai-type quasicrystals as well as approximants, we have investigated the phase constitution and phase stability in the Au-In-R(R=rare-earth) systems. In this research, alloys of various compositions were prepared by arc melting and were subsequently annealed under various conditions. The phase constitution and phase stability were investigated by X-ray diffraction and differential thermal analysis. As a result, we have observed the formation of 1/1 approximants in the Au-In-R systems and found that they are stable up to high temperatures of ~1000K. The formation condition and stability of icosahedral quasicrytals will be also reported in the presentation.

Synthesis and magnetic property of a Tsai-type 2/1 approximant

Various Tsai-type quasicrystals and approximants have been synthesized by replacing the constituent elements of the Cd-RE systems, based on the condition, (e/a)~2.0,. However, systems containing transition metals (TMs) have been rarely reported and, therefore, in this work we aimed to synthesize TM-bearing Tsai-type quasicrystals and approximants. Starting from the composition of the Ga-Pd-Gd 1/1 approximant [1], a search of TM-bearing approximants was performed by replacing Pd (1.376 Å) by Pt (1.378 Å). In this research, alloys of various compositions around the reported composition of 1/1 Ga-Pd-Gd were prepared by arc melting and they were then annealed at various conditions. The phase constitution was studied by X-ray diffraction and the thermal stability of the phases was examined by differential thermal analysis (DTA) for samples before and after annealing. As a result, we have observed formation of both 1/1 and 2/1 approximants with a=14.37 Å and 23.23 Å, respectively. The DTA curve exhibits no exothermic peak in the heating run up to the melting point (Tm=1180 K) for the 2/1 approximant, which suggests that the obtained 2/1 approximant is thermally stable up to Tm. The magnetic properties of the 1/1 and 2/1 approximants will be reported in the presentation.

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.

準結晶研究の最近の展開 ２．準結晶をつくる（準結晶の作製）新合金の準結晶：その構造と形成条件

Recent progress on a new group of icosahedral quasicrystals is reviewed. This group is characterized by Tsai-type atomic cluster as a local structural unit, which is different from Mackay and Bergman-types. This group can be regarded as largest including two Cd-based binary, seven Zn-based and ten Cd-based ternary stable phases as well as Cu- and Ag-based phases. They are all P-type, and some of them exhibit higher degree of structural perfection than the “perfect” Al-Pd-Mn quasicrystal. In the Tsai-type quasicrystals, there are substitution rules in the constituent elements. Hume-Rothery rules, namely the equality in e/a and in the ratio of atomic radii, play important role in the substitution rules.

New building blocks in the 2/1 crystalline approximant of a Bergman-type icosahedral quasicrystal.

The refined x-ray crystal structure of the phase Mg(27)Al(10.7(2))Zn(47.3(2)) (Pa3) establishes it as the new 2/1 Bergman-type approximant of the icosahedral quasicrystal. The primitive cubic lattice consists of condensed triacontahedral and novel prolate rhombohedral (PR) clusters. Each triacontahedron encapsulates the traditional, multiply endohedral Bergman-type clusters, and each PR encapsulates an Al(2) dimer. This phase exhibits the same long-range order as recently established for the Tsai-type Sc-Mg-Zn 2/1 approximant crystal, with substantial geometric and atomic distribution differences between the two only in the short range orders. This common feature suggests that Bergman- and Tsai-type quasicrystals may be more similar than earlier conceived. Factors germane to the formation of, and the differences between, Bergman- vs. Tsai-type 1/1 and 2/1 approximate structures are considered, including notably different distributions of the more electropositive elements.