Quasicrystals single quasicrystal

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

High-temperature liquid intermetallic solution growth of single crystals and quasicrystals... 

An authoritative review entitled The metal flux a preparative tool for the exploration of intermetallic compounds has been published by Kanatzidis etal. (2005). In this paper, containing a long list of references, several general and experimental aspects of this technique are discussed. The paper is enriched also by beautiful photographs of intermetallic single crystals, for instance a dodecahedral Ho-Mg-Zn quasicrystal grown from an Mg and Zn-rich flux. Special attention was dedicated to the use of molten metals as media (metallic fluxes) for the synthesis of different materials, and a number of key characteristics were underlined which the metal must possess in order to be a suitable flux. The following points were noticed ... 

Since then this method has been used to solve numerous other complex crystal structures [6-13]. Because solving a stmcture from a single projection requires a short (3 to 5 A) crystal axis, the method was later extended to combine the information from several orientations which allows also to uncover stmctures with pronounced overlap of the atom columns in projection. This technique was applied in 1990 to solve the 3D stmcture of the mineral staurolithe HFe2Al9Si404 [14, 15] and more recently to determine the stmcture of the huge quasicrystal approximant v-AlCrFe [16] which contains 129 atoms per as5mimetric unit. How CIP works to solve a crystal stmcture from projected data is shown in figure 10 (for further details see [17]). 

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). 

This model is a member of a series of structures, all built on a simple principle of interpenetration of polyhedra of five-fold point symmetry. The simplest member of the family is shown in Fig. (2.18(a)). Higher members, formed by successive interpenetrations of icosahedra fill space (Fig. 2.18(b)) and a perfectly ordered quasicrystal structure is obtained, there is no need to use oblate or prolate polyhedra they are generated by a single structure building principle [24]. 

Dubost, B., Lang, J-M., Tanaka, M., Saintfort, P., and Audier, M. Large Al-CuLi single quasicrystals with triacontahedral solidification morphology. Nature (London) 324, 48-50 (1986). 

It can be shown mathematically that five-fold axes cannot appear in a truly periodic crystal of single unit cells repeating in space. Nevertheless, some interesting quasicrystals have recently been discovered that have unusual symmetry properties. See Problem 3.39. 

Fig. 6 (A) A representative load-displacement curve of an indentation made at 3mN peak indentation load and (B) the hardness and elastic modulus as a function of indentation contact depth for the AI74 gCoig 9Ni8.4 quasicrystal. (Li, X. Zhang, L. Gao, H. Micro/nanomechanical characterization of a single decagonal AlCoNi quasicrystal. J. Phys. D Appl. Phys. 2004, 37, 753-757.)...

Both quasicrystals icosahedral Al-Pd-Mn (i-AlPdMn) and Al-Cu-Fe (i-AlCuFe) that belong to the F-phase are best modelled through a single model of the atomic positions that we introduce in this section. 

Ca ", when complexed by phyllosilicates, are associated with interlayer hydration, whereas K, Rb, and Cs tend not to be. X-ray diffraction data show that, when interlayer hydration occurs, x determined as indicated in Fig. 1.12 actually corresponds to a single layer of intercalated water molecules.Therefore, x for water vapor adsorption refers to monolayer coverage of the external surfaces plus one half of the accessible internal surfaces of the quasicrystals. This physical interpretation of for water vapor, as well as, that of jr, measured by nitrogen gas adsorption, permits... 

The application of LEED is limited to ultrahigh vacuum (UHV) conditions because of the character of the probe and its high surface sensitivity. The main application is the study of single crystal metal and semiconductor surfaces, and adsorption on these surfaces. Study of surfaces of thin insulator films is also possible. Recently, the application extends to study the surface structure of quasicrystals. LEED is widely applied to determine the symmetry and periodicity of surface structures and thus to identify reconstructions and adsorbate phases by observing the diffraction pattern. It is also a technique for quantitative surface structure analysis. LEED is also appHed as the quantitative determination of thermal vibrations at surfaces. 

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