Phosphate glasses with low dispersion and relatively high refractive indices (compared with silicate-based optical glasses) were developed for achromatic optical elements about 100 yr ago by Schott and co-workers. Subsequent interest in alkaline earth phosphate glasses stemmed from their high transparency for ultraviolet (UV) light, again when compared with silicate glasses. However, the poor chemical durability of these early optical glasses limited their applications and (temporarily) discouraged their further development. (See [1] for a description of these early studies and citations to them.)
In the 1950s, interest in amorphous alkali phosphates was stimulated by their use in a variety of industrial applications, including sequestering agents for hard water treatments and dispersants for clay processing and pigment manufacturing [2]. By studying such materials, Van Wazer [2] established the foundations for much of our present understanding of the nature of phosphate glasses. About the same time, Kordes and co-workers [3] and [4] re-examined the alkaline earth phosphate glasses, including UV-transmitting compositions, and noted some ‘anomalous’ trends in properties which they suggested showed a compositional-dependence for the coordination number of metal cations like Zn2+.
The advent of solid state lasers in the 1960s heralded a new era of phosphate glass research. Certain compositions have large rare-earth stimulated emission cross-sections and low thermo-optical coefficients (compared with silicate glasses) and are the materials of choice, particularly for high power laser applications [5]. (Campbell and Suratwala review the development of Nd-doped phosphate laser glasses elsewhere in this volume [6].)
More recently, phosphate glasses have been developed for a variety of specialty applications. Alkali aluminophosphate compositions have glass transition temperatures under 400°C and thermal expansion coefficients greater than 150×10−7/∘C and so are used for specialty hermetic seals [7]. Zinc phosphate compositions are chemically durable, have processing temperatures under 400°C and can be co-formed with high temperature polymers to produce unusual organic/inorganic composites [8]. The chemical durability and low processing temperature of iron phosphate glasses have led to their development as nuclear waste hosts [9]. Biocompatible phosphate glasses and glass-ceramics have medical applications [10] and amorphous lithium phosphate [11] and phosphorus oxynitride glasses [12] have fast ion conductivity that make them useful as solid state electrolytes.
The properties that make phosphate glasses candidates for so many different applications are related to their molecular-level structures. There have been many excellent reviews of structural studies on phosphate glasses, including those of Van Wazer [2], Abe [13] and Martin [14]. Since Martin’s 1991 review, a great deal of new structural information about phosphate glasses has been reported. For the first time, the structures of anhydrous ultraphosphate compositions have been determined, as have the structures of polyphosphate compositions with very low ( < 40 mol%) P2O5 contents. New structural probes, including solid state nuclear magnetic resonance (NMR) spectroscopy, X-ray absorption spectroscopy (XAS), and neutron diffraction analyses, have provided unprecedented detail about the bonding arrangements in glasses, from simple alkali and alkaline earth phosphates, through more complex and technologically useful, multicomponent glasses. These more recent studies, particularly those published since 1991, will be emphasized in this review.