elpejeno

The structure of liquids, glasses and other non-crystalline solids is characterized by the absence of long-range order which defines crystalline materials. Liquids and amorphous solids do, however, possess a rich and varied array of short to medium range order, which originates from chemical bonding and related interactions. Metallic glasses, for example, are typically well described by the dense random packing of hard spheres, whereas covalent systems, such as silicate glasses, have sparsely packed, strongly bound, tetrahedral network structures. These very different structures result in materials with very different physical properties and applications. The study of liquid and glass structure aims to gain insight into their behavior and physical properties, so that they can be understood, predicted and tailored for specific applications. Since the structure and resulting behavior of liquids and glasses is a complex many body problem, historically it has been too computationally inten...

The pair distribution function (or pair correlation function) of a material describes the probability of finding an atom at a separation r from another atom. A typical plot of g versus r of a liquid or glass shows a number of key features: At short separations (small r), g(r) = 0. This indicates the effective width of the atoms, which limits their distance of approach. A number of obvious peaks and troughs are present. These peaks indicate that the atoms pack around each other in 'shells' of nearest neighbors. Typically the 1st peak in g(r) is the strongest feature. This is due to the relatively strong chemical bonding and repulsion effects felt between neighboring atoms in the 1st shell. The attenuation of the peaks at increasing radial distances from the center indicates the decreasing degree of order from the center particle. This illustrates vividly the absence of "long-range order" in liquids and glasses. At long ranges, g(r) approaches a limiting value...

Other experimental techniques often employed to study the structure of glasses include Nuclear Magnetic Resonance (NMR), X-ray absorption fine structure (XAFS) and other spectroscopy methods including Raman spectroscopy. Experimental measurements can be combined with computer simulation methods, such as Reverse Monte Carlo (RMC) or molecular dynamics (MD) simulations, to obtain more complete and detailed description of the atomic structure.

Early theories relating to the structure of glass included the crystallite theory whereby glass is an aggregate of crystallites (extremely small crystals). However, structural determinations of vitreous SiO 2 and GeO 2 made by Warren and co-workers in the 1930s using x-ray diffraction showed the structure of glass to be typical of an amorphous solid In 1932 Zachariasen introduced the random network theory of glass in which the nature of bonding in the glass is the same as in the crystal but where the basic structural units in a glass are connected in a random manner in contrast to the periodic arrangement in a crystalline material. Despite the lack of long range order, the structure of glass does exhibit a high degree of ordering on short length scales due to the chemical bonding constraints in local atomic polyhedra. For example, the SiO 4 tetrahedra that form the fundamental structural units in silica glass represent a high degree of order, i.e. every silicon atom is coordinated ...

Egelstaff, P.A. (1994). An Introduction to the Liquid State . Oxford University Press. ISBN  978-0198517504 . Allen, M.P. & Tildersley, D.J. (1989). Computer Simulation of Liquids . Oxford University Press. ISBN  978-0198556459 . Fischer, H.E., Barnes, A.C., and Salmon, P.S. (2006). "Neutron and x-ray diffraction studies of liquids and glasses". Rep. Prog. Phys . 69 (1): 233–99. Bibcode:2006RPPh...69..233F. doi:10.1088/0034-4885/69/1/R05. CS1 maint: multiple names: authors list (link) Kawazoe,Y. and Waseda, Y. (2010). Structure and Properties of Aperiodic Materials . Springer. ISBN  978-3642056727 . CS1 maint: multiple names: authors list (link) Santen, L. & Krauth W. (2000). "Absence of a Thermodynamic Phase Transition in a Model Glass Former". Nature . 405 (6786): 550–1. arXiv: cond-mat/9912182 . Bibcode:2000Natur.405..550S. doi:10.1038/35014561. PMID 10850709.