Holistic Assessment of NIR-Emitting Nd3+-Activated Phosphate Glasses: A Structure–Property Relationship Study

Near-infrared (NIR)-emitting phosphate glasses containing Nd³⁺ ions are attractive for applications in laser materials and solar spectral converters. The composition–structure–property relation in this type of glass system is thus of interest from fundamental and applied perspectives. In this work, Nd³⁺-containing glasses were made by melting with 50P₂O₅-(50 – x)BaO-xNd₂O₃ (x = 0, 0.5, 1.0, 2.0, 3.0, 4.0 mol %) nominal compositions and studied comprehensively by density and related physical properties, X-ray diffraction (XRD), Raman spectroscopy, O 1s X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), dilatometry, ultraviolet–visible (UV–vis)–NIR optical absorption, and photoluminescence (PL) spectroscopy with decay dynamics assessment. The densities and molar volumes of the Nd³⁺-containing glasses generally increased with Nd₂O₃ concentration also resulting in shorter Nd³⁺–Nd³⁺ distances. XRD supported the amorphous nature of the glasses, whereas the Raman spectra evolution was indicative of glass depolymerization being induced by Nd³⁺ ions. Oxygen (1s) and phosphorus (2p) analysis by XPS for the glass with 4.0 mol % Nd₂O₃ agreed with the increase in nonbridging oxygens relative to the undoped host. DSC results showed that the glass transition temperatures increased with Nd³⁺ concentration, with the glasses also displaying a decreased tendency toward crystallization. Dilatometry showed trends of increasing softening temperatures and decreasing thermal expansion coefficients with increasing Nd₂O₃ content. A glass strengthening/tightening effect was then indicated to be induced by Nd³⁺ with higher field strength compared to Ba²⁺ ions. The UV–vis–NIR absorption by Nd³⁺ ions increased consistently with Nd³⁺ concentration. The UV–vis absorption edges of the Nd-containing glasses were also analyzed via Tauc and Urbach plots for comparison with the undoped host. Concerning the PL behavior, the Nd³⁺ NIR emission intensity was highest for 1.0 mol % Nd₂O₃ and decreased thereafter. The decay kinetics of the ⁴F_3/2 emitting state in Nd³⁺ ions analyzed revealed decreasing lifetimes where the decay rate analysis pointed to the prevalence of ion–ion excitation migration leading to PL quenching at high Nd³⁺ concentrations.