Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 186

The atomic arrangements that make up the structure of a condensed phase can be probed only with particles or electromagnetic radiation that penetrate into the material and interact with its atoms in such a way that analysis of the changes they undergo yield the specific information sought after. Probably the most basic information is relative atomic positions, which can be derived from X‐ray and neutron diffraction experiments because groups of atoms may act as gratings with respect to radiation whose wavelengths are in the Ångström‐range of interatomic distances. Alternatively, all other structural probes rely on the exchange of electromagnetic energy in certain frequency ranges to yield information not on the bulk structure of the material, but on individual types of atoms or groups of atoms. As a first example, absorption of X‐rays by electrons at eV‐energies probes the effects of interatomic bonding on the electronic levels of a given atom and, thus, the environment of this atom and even its redox state. As a second example, similar information may be derived for appropriate transition metal elements with Mössbauer spectroscopy from interactions at higher energies of gamma rays with atomic nuclei. A summary of the methods presented in this chapter along these lines is given in Table 1.

Regardless of the method used, probing atomic structure is much easier for crystals than for amorphous materials. To “solve” the structure, one only requires knowledge of the unit cell as defined by the crystallographic axes (a, b, c) and angles (α, β, γ), the lattice type (P, F, I, A, B, C, H, R), the symmetry associated with both the unit cell (point group) and the lattice (space group), and the positions of the atoms relative to the origin of the unit cell. One does not need to determine the position of all the atoms in the structure, but only the minimum number required by the point group symmetry.

Solving the structure of an amorphous material such as glass, on the other hand, currently is not possible and is unlikely in the foreseeable future because of the lack of long‐distance atomic periodicity. Hence, one cannot define a unit cell, a lattice, or their associated symmetry that would enable reproduction of the positions of atoms without having to determine the explicit location of all the atoms in three‐dimensional space. In essence, glasses have an infinite unit cell so that “solving” the structure would require knowledge of the position of every atom, an impossible task.

Nevertheless, it is possible to probe a great many structural features if one keeps in mind that the discrete values of interatomic angles and distances existing in crystals give way in glasses to continuous distributions that can be characterized by their shapes, widths, and maximum or averaged values. In addition to these parameters that characterize the bulk structure of the glass, information pertaining to specific atoms can also be gathered, such as the coordination polyhedra of the various constituents of the material.

Because the structure of a glass represents that of the solid “frozen‐in” at the glass transition, it is also important to investigate the marked temperature‐induced structural changes that take place in melts at higher temperatures. In this regard, two cases should be distinguished depending on the differences between the experimental timescale of the method and the rate of structural change of the melt. If the former is very short with respect to the latter, then the result will be a snapshot of a solid‐like structure; if it is long, the result will represent a time‐averaged structure that brings little information on the individual features that are averaged (see Section 4).

Table 1 Summary of techniques used to determine various features of glass structure.

Technique	Energy (eV)	Frequency (Hz)	Wavelength (nm)	Process	Information obtained
X‐ray diffraction	0.1–100 keV	3 × 1016–3 × 1019	0.01–10 nm	Scattering/diffraction from electrons	~0–15 Å, quantitative bond lengths and angles over short‐ and intermediate‐range length scales
Neutron diffraction	0.1–500 meV	0.04–120 THz	0.04–3 nm	Scattering/diffraction from neutrons	~0–15 Å, quantitative bond lengths and angles over short‐ and intermediate‐range length scales, dynamics
EXAFS	0.1–100 keV	3 × 1016–3 × 1019	0.01–10 nm	Atom‐specific absorption of X‐rays, multiple scattering	~5–6 Å, quantitative atom‐specific bond lengths and coordination
XANES	0.1–100 keV	3 × 1016–3 × 1019	0.01–10 nm	Atom‐specific electronic transitions to unoccupied orbitals, multiple scattering	~1–3 Å, qualitative coordination, oxidation state, electronic structure
XRS	Usually <10 keV	2.4 × 1013–3.4 × 1017	~1–1000 nm	Energy loss of inelastically scattered X‐ray photons	In‐situ high‐pressure energy loss spectra of low z elements (equivalent to XANES), short‐range structure, electronic structure
XPS	~0.1–1400 eV	2.4 × 1013–3.4 × 1017	~1–3000 nm	Energy of ejected core and valence electrons	Oxygen speciation (BO, NBO, free oxygen)
EELS/ELNES	10 meV–10 keV	7.2 × 1011–2.9 × 1017	~1–1000 nm	Energy loss of transmitted electrons through the sample	Same as XANES
IR	886 meV–3eV	430 × 1012–300 × 109	1 mm–2500 nm	Molecular vibrations	Vibrational states, quantification of CO2/H2O in glasses, coordination states, short‐ and intermediate‐range structure
Raman	1.2–120 meV	3 × 1011–1.5 × 1014	0.5 mm–1000 nm	Molecular vibrations, inelastic photon interactions	Vibrational states, Q species, ring statistics, short‐ and intermediate‐range structure
Brillouin	1.2 × 10−3–7.4 × 10−4 eV	107–1.8 × 1011	100–1000 nm	Inelastic photon–phonon interactions	Elastic and acoustic properties
UV/Vis	1.7–124 eV	30 × 1019–790 × 1012	10–700 nm	Valence electron transitions	Oxidation and coordination states of transition metals
NMR	12.4 peV–1.24 meV	3 × 103–300 × 109	~1 km–1 mm	Nuclear spin interactions	Short‐ and intermediate‐range structure, bond angles, coordination states, Q species, dynamics
Mössbauer	100 keV	>1019	<0.1 nm	Nuclear transitions	Oxidation state and coordination of Mössbauer active nuclei: typical Fe, Sn in glasses

In order to select the most suitable technique for probing the glass of interest, one has to determine what aspect of the structure is to be probed (bulk or atom specific), and what is the length scale to be probed (short‐range, intermediate‐range, or extended‐range)? In many instances more than one technique will need to be employed. I will briefly discuss a number of common and not so common techniques for deriving structural information on glasses. Whereas space constraints limit the detail that can be provided, I will provide a general overview of what aspect of the structure is probed and what information can be determined from each method. The reader should refer to any of a number of detailed reviews of the specific techniques currently available (e.g. [1, 2]).