Measurements of the refractive indices of ZnO single crystals are extended from room down to liquid-helium temperatures in the region between the absorption edge and 5500 Å for E⊥C and E∥C. The indices are calculated from the locations of interference maxima in transmission spectra caused by multiple reflections in thin (0.4–80 μ), plane-parallel platelets. The dispersion curves at room temperature are similar to those reported by Mollwo, but they are extended to include the optical isotropic point. At energies near absorption edge n∥<n⊥, while in the long wavelength region n∥>n⊥, as is found in other II-VI materials of the wurtzite symmetry. The isotropic point and its variation with temperature was also determined by birefringence measurements.
Basic polyhedral clusters have been derived from intermetallic compounds at near-eutectic composition by considering a dense packing and random arrangement of atoms at shell sites. Using such building units, bulk metallic glasses can be formed. Since the discovery of glassy systems, based on multicomponent alloys, in the early 1990s, bulk metallic glasses ͑BMGs͒ have been extensively studied because certain mechanical properties, such as strength, can be significantly improved over their crystalline counterparts. 1-11 BMGs produced so far usually contain three or more elements. 1-3 Until now, complex compositions have been considered necessary in order to inhibit crystallization of the liquid phase during cooling of the melt. However, simpler systems should be of great interest, fundamentally as well as technologically, since they would facilitate the atomic-structure determination for a given BMG, which has been a long-standing problem. [12][13][14][15][16][17][18][19][20][21][22][23][24] Originally, the dense-random-packed model was used to describe metallic glasses. 25 This model is based on the assumption that the glass consists of a random arrangement of spherical atoms ͑hard spheres͒ of each element. However, it has also been pointed out that localized, directional chemical bonding and the formation of groups of atoms are relevant factors to theories of glass formation and stability. 26 The local structure is well defined and similar to that of the crystalline form of the material. Recently, a topological model for metallic glass formation was proposed. 27,28 According to that model, a solute occupying either substitutional or interstitial sites in the host lattice can destabilize the lattice by producing a critical internal strain. Besides the structural models mentioned above, several criteria for structural stability of a BMG have been suggested: ͑1͒ large value of the reduced glass transition temperature T g / T l , where T g is the glass transition temperature and T l is the liquidus temperaure; 29 ͑2͒ three empirical rules for a large supercooled liquid region: ͑a͒ multicomponent system, ͑b͒ significant differences in atomic size, and ͑c͒ negative heats of mixing among the main constituent atoms; 1,3 ͑3͒ high gamma value, ␥ = T x / ͑T g + T l ͒, where T x is crystallization temperature; 30 ͑4͒ interaction between the Fermi surface and the Brillouin zone in a nearly-free-electron model; 31 ͑5͒ the critical concentration of a solute element required for amorphization decreases, reaches a minimum, and then increases with increasing ratio between the size of the solute and the solvent atoms; 27,32,33 ͑6͒ there is an optimum ratio R * between the size of the solute atom and the average size of the surrounding solvent atoms for dense packing. 28,34 In spite of the criteria mentioned above, the design of alloys with a high glass forming ability ͑GFA͒ remains to a large extent unpredictable due to lack of understanding of the local atomic structure. Although polyhedral clusters have been revealed in some met...
An experimental method of measuring absolute structure factors in imperfect single crystals by means of Bragg diffraction of 0.03 A y-radiation is presented. It is shown that in principle atomic scattering factorscan be deduced with an accuracy of about +0 5jo from rocking curves measured by means of y-ray diffractometry. Due to the uncertainties in the correction for thermal motion of the atoms, however, the values of the atomic scattering factors of copper at the 220, 222 and 333 reflexions could be derived only with an accuracy of _+ 1%. The measurements were made at room temperature.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.