A structural model for a quasicrystalline material (original) (raw)

Icosahedral AlCuFe alloys : towards ideal quasicrystals

Journal de Physique, 1990

2014 Le désordre topologique et l'ordre chimique à grande distance ont été étudiés par diffraction des rayons X sur poudre dans l'alliage i-Al65Cu20Fe15 brut de trempe. Le perfectionnement spectaculaire de l'état quasicristallin, obtenu par recuit d'échantillons hypertrempés, a été mesuré quantitativement ; le matériau recuit final correspond aux prévisions théoriques du modèle quasicristallin idéal dans les limites de la résolution instrumentale. Après recuit à 1 073 K, l'alliage de composition nominale Al65Cu20Fe15 est biphasé. La composition Al63Cu25Fe12 conduit quant à elle à une structure icosaédrique monophasée qui convient à l'analyse par diffraction des rayons X ou des neutrons sur un mono quasi-cristal. Abstract. 2014 The topological and chemical long range orders in the icosahedral Al65Cu20Fe15 alloy have been studied by X-ray powder diffraction. The spectacular enhancement of the quasicrystalline state by annealing of rapidly quenched samples have been quantitatively measured showing that the final annealed product fits the theoretical predictions of the ideal quasicrystal model within the instrument resolution. Whereas, after annealing at 1 073 K, Al65Cu2oFe15 is two-phased 2014 that makes difficult any fine study of the structure and the properties of the ideal icosahedral phase 2014 a close composition Al63Cu25Fe12 leads to a single-phased icosahedral structure suitable for single quasicrystal X-ray and neutron diffraction analysis.

Some new structural and electronic characteristics of quasicrystals

Bulletin of Materials Science, 1999

The quasicrystals being based on quasiperiodic order other than crystal like periodic translational order and embodying self similarity, present unique condensed matter phases. In addition to their curious structural characteristics the paucity of translational periodicity leads to drastic deviations in their electronic behaviour as compared to crystalline counterparts. This paper describes and discusses some new developments in regard to structural and electronic aspects of quasicrystalline materials. In regard to the structural aspects, two comparatively newer features will be described. One of them relates to the observation of variable strain approximants (VSA) first found in Ti68Fe26NiSi5, qc alloys; the other relates to the structure of decagonal phases. The variable strain approximants correspond to qc phases exhibiting variable strain for the different diffraction spots for the same reciprocal lattice row (possessing linear shifts). The VSA is thought to result from variable...

Recent research and development on Quasicrystals †

AIMS Materials Science, 2017

Quasicrystals are a kind of unique materials and characterized by the discrete diffraction patterns which cannot be indexed with the conventional three indices but require additional ones [1,2]. They occur in almost every type of solid including organic and inorganic compounds, minerals, metals and alloys, macromolecules and oxides [3,4,5]. It is convenient to describe their structures in higher dimensional space, a conceptual environment, in which threedimensional periodic crystals recover their periodicities [6]. The inconvenience of visualization due to extra dimensions is compensated by the mathematical elegance achieved through recovering periodicity. Studies of quasicrystalline structures have greatly enriched our understanding of the physics and chemistry of atomic orders, and have opened up new perspectives for correlating structure with the properties of complex materials. The ICQ13 conference was attended by 134 participants from all over the world including two participants from India, T.P. Yadav (Banaras Hindu University-Varanasi)

NMR and NQR study of the electronic and structural properties of Al-Cu-Fe and Al-Cu-Ru quasicrystals

Physical review. B, Condensed matter, 1994

2 7~1 and 6 3 , 6 5~~ NMR measurements are reported for powder samples of stable Al-Cu-Fe and Al-Cu-Ru icosahedral quasicrystals and their crystalline approximants, and for an Al-Pd-Mn singlegrain quasicrystal. Furthermore, ' '~1 NQR spectra at 4.2 K have been observed in the Al-Cu-Fe and Al-Cu-Ru samples. From the quadrupole-perturbed NMR spectra at different magnetic fields, and from the zero-field NQR spectra, a wide distribution of local electric-field gradient (EFG) tensor components and principal-axis-system orientations was found at the Al site. A model EFG calculation based on a 1/1 Al-Cu-Fe approximant was successful in explaining the observed NQR spectra. The average local gradient is largely determined by the p-electron wave function at the A1 site, while the width of the distribution is due to the lattice contribution to the EFG. Comparison of "CU NMR with 27Al NMR shows the EFG distribution at the two sites is similar, but the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more s-type wave function of the conduction electrons. The overall spread of EFG values is well reproduced by the calculation based on the approximant. However, the experimental spectra indicate a much larger number of nonequivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. The short-range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measurements of 27Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature yield results which indicate a reduction of the density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in the measured parameters were detected as a function of composition of the quasicrystatline alloys, arguing against a fine structure in the density of states at the Fermi level.

Mössbauer study of Al65Cu20Fe15 quasicrystals

Hyperfine Interactions, 1990

Al65Cu20Fe15 quasicrystalline ribbons exhibit similar paramagnetic spectra in the temperature range 45 mK-300 K, evidencing for no magnetic change down to 45 mK. Analysis of in field Mssbauer spectra by both negative and positive EFG distribution shows an only slightly dominant negative contribution (59%).

Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal

American Mineralogist, 2011

Icosahedrite, ideally Al 63 Cu 24 Fe 13 , is a new mineral from the Khatyrka River, southeastern Chukhotka, Russia. It occurs as dark gray-black anhedral to subhedral grains up to 100 µm across, closely associated with spinel, diopside, forsterite, nepheline, sodalite, corundum, stishovite, khatyrkite, cupalite, and an unnamed phase of composition AlCuFe. Icosahedrite is opaque with a metallic luster, possesses a gray streak, and is brittle with an uneven fracture. The density could not be determined. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. In plane-polarized incident light, icosahedrite exhibits neither bireflectance nor pleochroism. Between crossed polars, it is isotropic. Reflectance percentages (R min = R max ) for the four standard COM wavelengths are 62.3 (471.1 nm), 60.6 (548.3 nm), 58.1 (586.6 nm), and 56.0 (652.3 nm), respectively.