Structure and Properties of Phosphate Glasses

Recent development of phosphate glasses for variety of technological applications, from rare-earth hosts for solid state lasers to low temperature sealing glasses, have lead to renewal interest in understanding the structure and properties of these glasses. The properties that make phosphate glasses candidates for so many different applications are related to their molecular-level structures. In this review, spectroscopic studies of phosphate glasses, from simple binary through more complex multicomponent phosphate compositions, are discussed. The compositional dependencies of glass properties are explained by changes in the details of phosphate network structure. The effect of modifier has shown a systematic evolution of phosphate network from metaphosphate chains through the pyrophosphate structure to orthophosphate structure with increasing modifier content. Many phosphate glasses have a relatively poor chemical durability, but iron phosphate glasses are an exception. Family of iron phosphate glasses with an O/P ratio of 3.5 based on pyrophosphate (P2O7) groups is chemically very stable in aqueous solutions. Because of their high chemical durability, low melting temperature and high waste loading, iron phosphate glasses are of interest for nuclear waste immobilization. However, the basic knowledge of the structure-electrical properties of these glasses is of fundamental importance to understand the processing conditions, which could expand their applications. Phosphate glasses containing transition metal ions such as iron are electronically conducting glasses where the polaronic conduction is due to the electron hopping from low to high iron valence state. Special attention is given to the concentration of the charge carriers, ions and polarons, their interaction, magnitude of ionic an electronic conductivity as a consequence of the structural disorder in iron phosphate glasses doped with other metal ions. On the other hand, phosphate-based glasses offer a unique range of biomaterials, as they are soluble. Phosphate glasses are bioresorbable materials with solubility controlled by altering their chemical composition. Moreover, bioactive phosphate glasses form direct chemical bonding with hard/soft tissue. The surface charges of bioactive glasses are recognized to be the most important factors in determining biological responses. The improved bioactivity of the bioactive glasses as a result of the effects of the surface charges generated by electrical polarization is discussed.