Stable quasicrystals

Photoinduced, spontaneous aggregation processes have been shown to occur when indolinobenzopyrans are irradiated in aliphatic solvents. The aggregates which are globular in appearance, consist of submicron cores of crystalline materials with an amorphous exterior and are termed "quasicrystals" (L-3). Spectroscopic studies by Krongauz and coworkers (1) indicate that the composition of the cores are AnB (n=2,3) and the amorphous exteriors AB. The most stable quasicrystals have been derived from 1-(/ -metha-crolyloxyethyl)-3,3 dimethyl-61-nitrospiro- (indoline-2,2 —[2H-1 ] benzopyran (SP-A) and its associated merocyanine form (SP-B). 

The entire area has experienced a rebirth in the last few years, for several reasons. The most important perhaps is the realization that cuprate superconductors are not far from a largely correlation driven metal-insulator transition. Another is the appreciation that the transition is of wide occurrence, some recent examples being the superconductor-insulator, the quantum Hall fluid-insulator, and the stable quasicrystal metal-insulator transitions. Hopefully, the next few years should see considerable progress in the experimental and theoretical description of this basic electronic transition. 

As the first reported quasicrystals were metastable phases at room temperature produced by rapid solidification, they were consequently of poor quality. Stable quasicrystals have since been discovered that have revealed very high strucmral perfection, even comparable to single crystals. This discovery made it possible to apply conventional solidification techniques. The preferred method appears to be system-specific, as it depends on the temperature stability of the quasicrystalline phase. If the quasicrystal is only stable at elevated temperatures, for example, it can decompose into a crystalline phase if the melt is solidified slowly. If the phase is thermodynamically stable down to room temperature, as is the case for Al-Pd-Mn, quasicrystals can be grown with conventional cooling rates (e.g. 10°C/h). 

Industry is highly interested in new materials that possess new or improved properties. The use of structure maps and other diagrammatic tools aid in this quest. Below are described the uses of such maps in the search for property-specific materials. In the sections on stable quasicrystals, high-Tc superconductors, and ferroelectrics, the applications of the quantum structural diagrams (QSD) described above will be used. In these classes of materials, the analysis starts with the compilation of the diagrammatic coordinates as described in equation (6). For these unusual types of order, the existence of diagrammatic regularities implies, at least, that the same factors that control local structure and stability in ordinary compounds also determine the tendency to these types of order and probably a coimection as well. 

All known stable quasicrystals are either icosahedral or decagonal. The experimentally best studied quasicrystal is the large monograin of icosahedral Al-Pd-Mn, (Al7oPd2iMn9) [1], followed by icosahedral Al-Cu-Fe (Al62Cu25.5Fei2.5) [2], Both quasicrystals belong to the icosahedral F-phase. 

Rabe KM, PhUlips JC, VUlars P, Brown ID (1992) Global multinary stmctural chemistry of stable quasicrystals, high-Tc ferroelectrics, and high-Tc superconductors. Phys Rev B45 7650-7676... 

Icosahedral quasicrystals. Many icosahedral phases (both stable and metastable) have been identified in several systems, mainly Al-based but also Mg-based, Ti-based, etc. 

Figure 3.50. The Yb-Cd binary phase diagram. The congruently melting compound YbCd57 is the first stable binary quasicrystal identified.

Steurer, W.(2005) Stable clusters in quasicrystals -fact or fiction Proceedings of the 9th International Conference on Quasicrystals, Ames, 22-26 May, http //www.icq9.ames-lab.gov... 

Since the initial discovery of metastable quasicrystals, many ternary inter-metallic compounds have been produced in the quasicrystalline state, which are thermodynamically stable at room temperature. These have been obtained... 

Some stable ternary intermetallic phases have been found that are quasiperi-odic in two dimensions and periodic in the third. These are from the systems Al—Ni—Co, Al—Cu—Co, and Al—Mn—Pd. They contain decagonally packed groups of atoms (local tenfold rotational symmetry). It should be noted that there are also known metastable quasicrystals with local eightfold rotational symmetry (octagonal) and 12-fold rotational symmetry (dodecagonal) as well. The dodecahedron is also one of the five Platonic solids (Lalena and Cleary, 2005). 

It turns out that icosahedral coordination (Z = 12), and other coordination polytetrahedra with coordination numbers Z= 14, 15, and 16, are a major component of some liquid stmctures, more stable than a close-packed one, as was demonstrated by F. C. Frank and J. Kasper. When these liquid stmctures are rapidly solidified, the resultant stmcture has icoshedra threaded by a network of wedge discUnations, having resisted reconstmction into crystaUine units with three-dimensional translational periodicity (Mackay, 1985 TumbuU, 2000). Stable ternary intermetaUic icosahedral quasicrystals... 

These characteristics of quasicrystal formation and breakdown are consistent with the trends in Fig. 6.2. When Enh is less than 0.1, for example, the properties of a mixed Na/Ca-montmorillonite do not differ much from those of Ca-montmorillonite and the quasicrystal should remain a stable entity. When Ens is larger than 0.6, however, the mixed Na/Ca-montmorillonite exhibits properties that are indistinguishable from those of a Na-montmorillonite and a quasicrystal should be an inherently unstable structural unit. 

The chemistry and structures of SAMs of alkanethiolates on gold have been extensively studied, and need not be reviewed here [44]. In brief, when fully equilibrated and in their most stable form, these SAMs seem to be two-dimensional quasicrystals, with the sulfur headgroups epitaxial on the gold surface. A 30° tilt of the trans-extended alkane chains brings these chains into van der Waals contact. Functional groups present on the termini of the chains are exposed to the solution. Conformational disorder in the system is concentrated in the terminal regions of the chains. 

There are three known thermodynamically stable blue phases BPi and BPjj, and BPin (or the foggy phase). The structure of the first two is already established BPj is a body-centered cubic phase (symmetry group O or /4i32) and BPu is simple cubic (symmetry group or P4232) [42]. The foggy phase, BPm, can probably be described using one of the quasicrystal models [43]. 

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