Glasses: Phosphate glasses
A category of glass based on phosphorus pentoxide (P2O5), phosphate glasses are formed with a structure of phosphorus tetrahedra linked by bridging oxygens - as shown in the bottom image above. These tetrahedra are classified using Qt terminology, where t is the number of bridging oxygens per tetrahedra. (Q0 is an individual orthophosphate anion, while Q3 is a phosphate tetrahedron with three bridging oxygens connecting it to the rest of the glass structure).
This network of tetrahedra is easily interrupted, meaning that, in general, phosphate glasses have very low glass transformation temperatures (meaning they can be formed at lower temperatures). The structure of the phosphorus pentoxide used to create the glass (of which the crystalline form has several polymorphs) also has an effect on the structure of the glass, and thus its properties. Glasses produced with short melting times retain some of the structural details of the crystalline forms, though longer melting times allow the properties to converge toward equilibrium values. Phosphate glasses are also typically softer than the more common silicate glasses, with poor chemical durability, high thermal expansion coefficients, and the added bonus of being biocompatible.
Just as with other glasses, elements added to phosphate glasses can alter properties in favorable ways. The addition of alkali and alkaline earth oxides break the structural rings and convert the glass network to linear chains of phosphorus-oxygen tetrahedra - if these chains are oriented during fiber drawing, phosphate glasses with directional properties can be produced, resembling organic polymeric glasses. Replacing some of the oxygen with nitrogen in phosphate glasses can both improve the chemical durability and increase the glass transformation temperature.
Phosphate glass is often used as laser gain media, thanks to its high solubility for rare earth ions such as erbium, ytterbium, and neodymium, and is being considered as a possible waste matrix to immobilize high level nuclear wastes, thanks to its higher waste loading capacity and lower processing temperatures. Phosphate glasses can also be mixed with other types of glass, yielding glasses such as fluorophosphate, phosphosilicate, and aluminophosphate glasses. Other applications of phosphate glasses include fast ion conductors, glass-to-metal seals, and biomedical engineering.
Sources: ( 1 ) ( 2 - top middle image ) ( 3 ) ( 4 ) ( 5 )
Image sources: (Top left) (Top right) (Bottom)
