An NMR and Raman spectroscopy study of Li₂O–SrO–Nb₂O₅–P₂O₅ glasses

A new Li–Sr–Nb phosphate series of glasses, Li₂O–SrO–Nb₂O₅–P₂O₅, containing 0–40 mol% Li₂O and 10–30 mol% Nb₂O₅ was prepared, and their structural properties, thermal properties and aqueous durability were systematically investigated. With addition of 0–30 mol% Li₂O, the Q² dominates the glass network accompanied with a few Q¹ and Q³ tetrahedral units. However, for 40 mol% Li₂O addition, Li⁺ cations depolymerize the P–O–P bonds on Q³ sites, resulting in the decrease in the cross-linking density and strength of the structure. Nb₂O₅ in our glass systems not only acts as a modifier but also as a glass former. The addition of Nb₂O₅ in the vitreous (60 − y)P₂O₅–yNb₂O₅–20SrO–20Li₂O system creates a stronger P–O–Nb bonding to substitute P–O–P and O–P–O bonds evidenced by the Raman spectra, and eventually (Nb–O–Nb)cₕₐᵢₙ and (Nb–O–Nb)₃D are dominant in the structure. Thus the Nb₂O₅ composition effect shows a systematic trend in the enhancement of the network structure. Moreover, the composition effect on the distribution of Qⁱ tetrahedral units was studied by NMR analysis in conjunction with deconvoluted Raman results. The effects of Li₂O and Nb₂O₅ composition on the glass transition temperature, glass dilatometric softening temperature and coefficient of thermal expansion were also investigated. Experimental results reveal that the glass transition temperature and aqueous durability are considerably increased with increasing Nb₂O₅ content, but decreased with increasing Li₂O content. The coefficient of thermal expansion is found to increase with increasing Li₂O content. Lower Nb₂O₅ content, i.e., higher P₂O₅ content, in the Li–Sr–Nb phosphate serial glasses results in higher coefficient of thermal expansion, but the glasses still exhibit great structure stability. The aqueous durability is determined from the dissolution rate, which is strongly dependent on the Nb₂O₅ composition. The lowest dissolution rate is about 5 × 10⁻⁹ g cm⁻¹ min⁻¹, ensuring that the 30P₂O₅–30Nb₂O₅–(40 − x)SrO–xLi₂O vitreous system demonstrates excellent aqueous durability.