Vitreous State of Low-Molecular-Weight Compounds

the solid amorphous state of a substance formed upon the hardening of the substance’s supercooled melt. The reversibility of the transition between the vitreous state and the melt is a feature that, in addition to the mode of preparation, distinguishes the vitreous state from other solid amorphous states, in particular, thin amorphous metal films. The gradual increase in the viscosity of a melt hinders the crystallization of the substance, that is, transition to a solid state having minimum free energy. For example, the coefficient of dynamic viscosity of a glass-forming substance like SiO₂ at the melting point (1710°C) is 10⁷·⁷ poises; in contrast, the coefficient for water at the melting point (0°) is 0.02 poise. The transition of the melt to the vitreous state, referred to as vitrification occurs within a certain temperature range. The vitreous state is metastable; the transition of a substance from the vitreous state to a crystalline state is a first-order phase transition.

A significant number of inorganic substances can exist in the vitreous state, including elements (S, Se, As, P), oxides (B₂O₃, SiO₂, GeO₂, As₂O₃, Sb₂O₃, FeO₂, V₂O₅), aqueous solutions of H₂O₂, H₂SO₄, H₃PO₄, HClO₄, H₂SeO₄, H₂CrO₄, NH₄OH, KOH, HCl, and LiCl, chalcogenides of arsenic, germanium, and phosphorus, and certain halides and carbonates. Many of these substances form the basis of various types of complex glass.

A substance in the vitreous state manifests itself through a rigid system of atoms and groups of atoms, the bonds between which are to a greater or lesser extent covalent. Diffraction studies (X-ray diffraction analysis, electron diffraction, neutron diffraction) permit a determination of the arrangement of adjacent atoms (short-range order). A radial distribution curve is constructed by measuring the radii and amplitudes of the diffraction maxima. The maxima of this curve correspond to the interatomic distances, and the area bounded by the maxima provides information on the average number of atoms immediately adjacent to a given atom.

Substances in the vitreous state are isotropic and brittle, and they display conchoidal fracture upon shearing. Depending on the composition, they are transparent in certain regions of the spectrum (visible, infrared, ultraviolet, X-ray, γ-ray). Mechanical stresses (arising from poor annealing) and nonuniformity in the structure of a substance in the vitreous state figure as causes of double refraction. This type of refraction, because of the uncontrolled factors that cause it, is unstable and harmful in optics. However, double refraction, which is caused by electric and magnetic fields, is not without certain uses. Practically all types of glass exhibit luminescence to a slight extent, and activators, such as rare-earth elements and uranium, are added to glass to enhance this effect. Strong coherent radiation is obtained through pumping and the use of specially selected activators. Substances in the vitreous state, as a rule, are diamagnetic; significant admixtures of oxides of rare-earth metals make substances in the vitreous state paramagnetic. Ferromagnetic materials, for example, certain sitalls, are obtained from glass having a special composition. In their electrical properties, most types of glass are dielectrics (vitreous silicas), but there are many substances that in the vitreous state possess semiconductor properties (chalcogenide glass).