Alkaline Earth Metal Fluorides (MgF ₂ , CaF ₂ , SrF ₂ , BaF ₂ ) and Nb ₂ O ₅ Effect on the Structural and Optical Properties of New Fluorophosphoniobate Glasses

Niobium-fluorophosphate glasses show promising technological and scientific potential in a wide range of optical and photonic applications due to their properties as a host matrix, such as wide transparency between the ultraviolet and near-infrared, high solubility to rare-earth ions, low phonon energy, and high chemical stability. Efforts were previously made to study the effects that different concentrations of niobium oxide have on the base phosphate glass composition used here in the structural, thermal, and optical properties. However, an exploration of which changes different alkaline earth metals can induce in niobium-phosphate glass properties, considering their modifying role and periodic properties, is lacking. Therefore, this study aimed to thoroughly investigate how different alkaline earth metals can induce variations in the structural, thermal, and optical properties of a novel niobium-phosphate glass. The tested glasses followed the compositional rule (80 - <i>y</i>)-NaPO₃-<i>y</i>Nb₂O₅-20XF₂ (X = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, <i>y</i> = 5, 10, 15, 20 mol % of Nb₂O₅) and were synthesized by the melt-quenching method. Analysis by differential scanning calorimetry (DSC), UV-vis absorption spectroscopy, and optical bandgap calculations shows that the covalent character of the glass matrix increases for increasing Nb₂O₅ content, causing an increase in the glass transition temperature, <i>T</i> _g, and a decrease of the optical bandgap energy. DSC analyses revealed a very high stability against crystallization, Δ<i>T</i> up to nearly 400 °C (Δ<i>T</i> = <i>T</i> _x - <i>T</i> _g)where <i>T</i> _x is the crystallization peak temperaturefor this glass-forming system. ³¹P NMR experiments revealed that the increase in Nb₂O₅ between 5 and 15 mol % induced the formation of P⁰, P_1Nb ¹, and P_2Nb ² phosphate units, consequently increasing the glass matrix connectivity due to the formation of P-O-Nb and Nb-O-Nb bonds. Moreover, ¹⁹F nuclear magnetic resonance showed how the alkaline earth metals with a higher charge-to-radius ratio (smaller ionic radius) preferentially bond with the fluoride species within the glass matrix. Consequently, the glass connectivity increases due to the lower availability of fluoride to interact with the main glass former groups (i.e., phosphate and niobate groups).