Q-species in Alkali-disilicate Glasses

Glass, in contrary to crystals, lacks long-range translation symmetry so that standard experimental scattering methods such as X-ray or neutron scattering must be improved and/or sophisticated both theoretically and experimentally to obtain comparable results. However, scattering experiments are not able to provide 3D image of the structure but only its 1D projection resulting in the so-called radial distribution function (RDF). RDF makes possible (although not unambiguously) to determine short-range arrangement in glass structure. This short-range order (SRO) is approximately described by RDF up to 3.3 [1]. Unfortunately, many glass properties are associated with mediumrange order (MRO) which ranges from 3.3 up to approximately 10 (figure 1). Exploration of MRO in silicate glasses may be conveniently performed by means of Q-species and their distributions [2]. Distributions of Q-species are experimentally available by nuclear magnetic resonance methods [3 9] and Raman spectroscopy [10 12]. Q distinguishes silicon atoms according to the number n of coordinated bridging oxygens (BO), i.e. Q stands for the silicon coordinated by four BO, Q corresponds to three BO and one nonbridging oxygen (NBO), etc. In silica glass all oxygen atoms are assumed to be BO. By introducing alkali oxides into silica structure some Si-BO-Si interconnections are broken and new bonds of Si-NBO type are created. The Q-species distributions of alkali ions in silicate structures generally depend on species of alkali ion and the temperature (above Tg) [5, 10, 13]. Smaller alkali ions tend to form alkali-rich regions, what may result in the increasing amount of Q. In these alkali-rich regions, chain-like structures of Q are assumed, while Q dominate in silica-rich regions [2, 4, 8]. On contrary, larger ions are rather uniformly distributed, so that Q species (sheet-like structures) are preferred in such glass. Q-species balance equation (1)