Glass Forming Systems Under Investigation

-  Lead Silicate PbO-SiO₂

Silica, SiO₂, is the archetypal network glass forming material, with silicate glasses having been produced for thousands of years.  Soda-lime-silica (Na₂O-CaO-SiO₂) glass for example is the basic system used in a huge range of practical applications, such as modern window panes, produced by the float glass method.  Lead silicate on the other hand is the base system from which lead crystal glass is composed.  The properties imparted by the lone pair Pb²⁺ cations include high refractive indices, low glass transition and working temperatures and low wavelength IR absorption cutoffs, among other important optical properties.  The system has been studied extensively, with the first study of the atomic structure conducted by Bair in 1936.  Despite the intensive examination of such glasses, there remains some contovosy over the structural role and coordination about the lead cations, as well as their medium range ordering in the glass network.  As such a detailed structural study by neutron and  x-ray diffraction, alongside numerous spectroscopic measurements is to be conducted and complemented by computer modelling techniques such as EPSR.

- Lead Germanate PbO-GeO₂

Whilst pure vitreous germania, GeO₂, is structurally analogous to the random network structure of pure glassy SiO₂, binary oxide germantes are found to differ significantly from their silicious counterparts.  This is manifest in the germanate anomaly, which refers to the observed extrema in various physical properties as a function of binary oxide glass composition.  The most widely accepted explanation of this phenomenon is the induced increase in coordination number of germanium to oxygen as modifier oxide is introduced to the glass.  Once the network is saturated with higher coordinated Ge (due to the repulsion between negetively charged [GeO_n], n>4, polyhedra), it begins to break up, with increased modifier content, and the average Ge-O coordination number decreases, leading to maxima in the measured bulk densities.  The interplay between the local environments of the Ge and Pb cations is of great interest in this system.

- Alakaline Earth Germanates AeO-GeO₂

Whilst the alkali germanates, A₂O-GeO₂, have garnered considerable interest, the alkaline earth systems have not been examined in any great detail.  It is of basic importance to understand glass structure in a wide variety of systems, and furthermore, it is expected that the determination of local structural arrangements in Ca, Sr and Ba bearing germanate glasses will be invaluable in the elucidation of the structure in the lead germanate system.

- Barium Diborate BaO.2B₂O₃

B₂O₃ is another important glass-forming oxide, with borosilicate based systems used commercially as low thermal-expansion materials such as Pyrex.  Borate glasses are fundamentally of interest, owing to the medium range ordering present in many cases, manifest in the form of superstructural units.  For example, pure vitreous boron oxide is composed of a random netork of trigonal planar [BO₃] units along with boroxol rings composed of three such units.  The concentration of such rings, and whether or not it is above the statistically expected concentration remains a contentious issue.  Introduction of modifying oxides to a borate system results in a borate anomaly, c.f. the germanate anomaly above.  This is due the introduction of boron in tetrahedral sites, which charge compensates the modifying cations, until saturation where the boron-oxygen network begins to break up.  The reasons for studying the BaB₄O₇ stoichiometric glass are numerous.  For one it is strongly polymorphic, and the devitrification process can lead to different polymorphs, determined by thermal history.  This is to be studied by differential thermal analysis and high temperature x-ray diffraction, among other techniques.  Furthermore, the reported thermal expansion behaviour is of interest.  The system additionally acts as a case study for the general problem of relating crystal and glass structure.  This is of course difficult when crystal structures are unknown, however, the use of solid state ¹¹B NMR techniques can be of great use here.

The GEneral Materials (GEM) diffractometer, at the ISIS spallation neutron source, UK