The techniques of neutron diffraction and x-ray diffraction, as applied to structural studies of liquids and glasses, are reviewed. Emphasis is placed on the explanation and discussion of the experimental techniques and data analysis methods, as illustrated by the results of representative experiments. The disordered, isotropic nature of the structure of liquids and glasses leads to special considerations and certain difficulties when neutron and x-ray diffraction techniques are applied, especially when used in combination on the same system. Recent progress in experimental technique, as well as in data analysis and computer simulation, has motivated the writing of this review.

Neutron and x-ray diffraction studies of liquids and glasses

Publisher Summary Metallic glasses are solids that have electronic properties normally associated with metals, but with atomic arrangements that are not spatially periodic. Noncrystalline and amorphous are equivalent terms used to describe the atomic scale structure of such materials. The term glass has often been reserved for amorphous solids formed by continuous solidification of a liquid, but is used in this review to refer to amorphous solids produced in a variety of ways. These include evaporation, sputtering, and electro- and chemical deposition, as well as quenching from the liquid state. The current status of research on structure of metallic glasses is reviewed in this chapter. Discussion is largely limited to metallic glasses which can be retained at room temperature. All of these contain at least two atomic components. Nominally pure, elemental metallic glasses have been prepared, but their metastability depends critically on impurity content; most of these crystallize well below room temperature.

Structure of Metallic Alloy Glasses

X‐ray diffraction data show that there are at least two forms of amorphous solid water which differ in density and second nearest‐neighbor oxygen–oxygen distribution. (a) The lower density form, made at 77 °K, has a diffraction pattern consistent with a structure that has oxygen–oxygen nearest‐neighbor tetrahedral symmetry on average, and a nearest neighbor O–O separation of 2.76 A with small dispersion. The density of this material is estimated to be 0.94 g cm−3. While it is not possible to uniquely define the structure, the data available support the notion that its fundamental characteristic is the existence of a randomized network of hydrogen bonds with O–O–O angular distribution derived from (i.e., centered about) that of ice Ih. Comparison of neutron diffraction and x‐ray diffraction data suggests strongly that the first shell hydrogen bonds are nearly linear and that orientational correlations between water molecules are limited to nearest neighbors. (b) The higher density form, made at 10 °K, has a diffraction pattern similar to, yet distinctively different from, that of the high temperature deposit. The O–O nearest neighbor distance is the same, 2.76 A, but the dispersion in this separation is larger in the low temperature form. The diffraction pattern shows an extra peak at 3.3 A, corresponding to about 1.4 molecules, the existence of which is responsible for the estimated higher density, namely 1.1 g cm−3. The data are consistent with several models which share the feature of introducing small O–O–O angles into the structure. We discuss the relationships between our data, and inferences from the data, and the corresponding data for liquid water.

Diffraction pattern and structure of amorphous solid water at 10 and 77 °K

The x‐ray diffraction patterns of liquid methanol and ethanol have been measured at 20 °C. The data are analyzed to yield the molecular structures, and the distinct structure functions Hd(k) are analyzed to obtain the hydrogen bonding in these alcohols. The data show clearly that hydrogen‐bonded hydroxyl groups occur in methanol and ethanol with an OH⋅⋅⋅OH distance of 2.8A, and that each hydroxyl group has 1.8±0.1 nearest neighbors at this distance.

Hydrogen bonding in liquid methanol and ethanol determined by x‐ray diffraction

The rare earth complex,[RE(H3O)8]3+,has been identified in concentrated (3.2−3.6 m) aqueous TbCl3,DyCl3,ErCl3,TmCl3, and LuCl3 solutions from x ray diffraction measurements. The inner sphere water coordination of rare earth ion was obtained from quantitative resolution of the radial distribution functions. In each solution the rare earth ion has eight water nearest neighbors with the average RE3−H2O distances being 2.409, 2.396, 2.369, 2.358, and 2.338A for TbCl3,DyCl3, ErCl3, TmCl3, and LuCl3,respectively. The average RE3+. . . Cl− ion pair distances are near 4.8A.

The coordination (hydration) of rare earth ions in aqueous chloride solutions from x ray diffraction. I.TbCl3,DyCl3, ErCl3,TmCl3,and LuCl3

A rigorous treatment of the multiple scattering of x-rays in amorphous samples has been developed in connection with the x-ray studies of the structure of simple glasses. A paper concerning this research has been submitted for publication to Acta Crystallographica.

R. L. Mozzi

Papers

Multiple scattering of X-rays by amorphous samples