Preparation and characterization of new niobophosphate glasses in the Li2O-Nb2O5-CaO-P2O5 system

In the present work we describe the synthesis, spectroscopy, thermal and chemical durability properties of the vitreous system Li2O-Nb2O5-CaO-P2O5 (LNCP). Investigations of the short-range order by Fourier transform infrared, Raman, UV-VIS and 31P MAS-NMR spectroscopies suggest that the network former glass consists of Nb octahedra linked to pyro/orthophosphate units through Nb—O—P bonds. The presence of modifier cations (Li+ and Ca2+) promotes depolymerization of the P—O—P chains, yielding pyro/orthophosphate units. The presence of this kind of structure accounts for the improvement of the chemical durability at low pH when the Nb content in the LNCP glass composition is high. The density and linear refractive indices of LNCP glasses increased linearly as the Nb2O5/P2O5 molar ratio increased, as a consequence of P2O5 substitution by Nb2O5 as the glassformer. The dependence of the glass transition temperature, the softening temperature and the crystallization temperature on the Nb2O5/P2O5 ratio exhibits the same behavior. On the other hand, the thermal expansion coefficient decreases with the increased Nb2O5/P2O5 ratio.