Zachariasen [18] identified vitreous P2O5 as one of the prototypical ‘random network’ glass formers. Indeed, recent diffraction studies of v-P2O5, described below, are consistent with the Zachariasen description of an open, distorted network of Q3 tetrahedra. Hägg [19], however, challenged Zachariasen’s idea that glass forming tendency relied upon the development of a random, three-dimensional network, using the metaphosphate composition as an example of glass formation in which a large one-dimensional ‘molecular’ group (based on Q2 tetrahedra) inhibits crystallization. The chromatography studies of Van Wazer [2] and others since have reinforced the utility of the description of phosphate glass structures in terms of distributions of molecular species such as those proposed by Hägg.
These two descriptions of glass structure are not incompatible; both predict local bonding similarities in glasses and crystals of identical composition. (That is, a metaphosphate glass and crystal will both have structures based on linkages of Q2 tetrahedra.) More importantly for the present review, these two models also illustrate the need to describe ‘glass structure’ at different length scales. Having information about the local bond arrangements of the glass forming (or modifying) polyhedra will not necessarily complete the description of a glass structure. That short-range information (generally defined by the immediate coordination environment of an ion; e.g., see [20]) must also be coupled with longer range information, including how those polyhedra are linked to form larger structures, whether ordered or disordered, before a proper structural description is available. The studies reviewed below describe both the short-range and longer-range structures of phosphate glasses.
Phosphate glasses can be made with a range of structures, from a cross-linked network of Q3 tetrahedra (vitreous P2O5) to polymer-like metaphosphate chains of Q2 tetrahedra to ‘invert’ glasses based on small pyro- (Q1) and orthophosphate (Q0) anions, depending on the [O]/[P] ratio as set by glass composition. This review is likewise organized in this order.