Structural investigations of phosphate glasses: a detailed infrared study of the x(PbO)-(1?x) P2O5 vitreous system

The results and detailed discussion of an extensive experimental study of infrared spectra of the x (PbO)-(1–x)P₂O₅ vitreous system (x=0.3–0.75) together with a brief review of infrared spectra of phosphate compounds, are presented. Theoretical models employed in the interpretation of infrared spectra of glasses have been reviewed. The frequency ranges of various infrared bands belonging to PO ₄ ^3– and P₂O ₇ ^4– , observed in different phosphate compounds, are discussed. The glassy and quenched samples were prepared from PbO and NH₄H₂PO₄ by the rapid quenching technique. The infrared spectra of the constituents of the system, PbO and P₂O₅, in their polycrystalline and glassy forms, have been discussed. The intensity and wavenumbers of the infrared bands around 1600 and 3300 cm^–1, assigned to the bending and stretching modes in H₂O trapped by the hygroscopic glasses, have been followed for different compositions with x<0.5. The changes observed in these infrared bands established the role of water as an additional glass modifier. The intensity and frequency variations of the infrared bands have been followed through all the compositions for characteristic phosphate group frequencies including P=O, P-O-P stretching and bending modes and P-O bending mode. The results clearly suggest that the x(PbO)-(1–x)P₂O₅ system undergoes gradual structural changes from metaphosphate (x=0.5), to pyrophosphate (x=0.66) and to orthophosphate (x=0.75). The continuing presence of the infrared band, in varying intensity, in the region 1200–1280 cm^–1 attributed to P=O, suggests that the glass-forming ability of the binary system is extendable at least up to x=0.66 composition, and that no complete rupture of P=O bond by Pb²⁺ takes place. The ionic character of the phosphate groups, P-O^(–), PO ₄ ^3– is well revealed by significant changes with the PbO content in the spectral features of the infrared bands around 1120 and 980 cm^–1 respectively. The maximum intensity of the P-O^(–) band at 1120 cm^–1 for 55 mol% PbO suggests a partial breakdown of the covalent vitreous network of the phosphates and formation of a crystalline phase consisting of ionic groups PO ₄ ^3– , P₂O ₆ ^2– and P₂O ₇ ^4– for PbO greater than 55 mol%. The observed pattern of variation in the intensity of the infrared bands in the 940–1080 cm^–1 region attributed to the v₃-mode in PO ₄ ^3– , suggests a gradual transformation of PO ₄ ^3– units to PO ₃ ^– groups in lead meta-phosphate glass and then their restoration to PO ₄ ^3– groups of pyro- and ortho-phosphate quenched samples. The results indicate a gradual decrease in the number of bridging oxygens and increase in the resonance behaviour of non-bridging oxygens as the mole percentage of metal oxide (PbO) increases in the glass. The infrared spectra of several binary phosphate glasses have been reviewed in the context of the study of effect of the cation on the infrared spectra. It is found that the influence of the cation on the infrared spectra of phosphate glasses does not show any striking regularity. Theoretical calculations of these band frequencies were found to agree well only in the case of pure stretching (P=O and O-H) vibrations and pure bending (P-O-P and O-H) vibrations. The disagreement in the case of P-O^(–), P-O-H and other modes of P-O-P groups, has been attributed to the mixed nature of modes occurring in glasses. The changes in the positions of the characteristic bands and their relative intensities are strongly dependent on the structural units and PbO content in the phosphate glasses and the results emphasize the role of PbO as a network modifier.