Holistic Assessment of NIR-Emitting Nd 3+ -Activated Phosphate Glasses: A Structure-Property Relationship Study.

Near-infrared (NIR)-emitting phosphate glasses containing Nd 3+ 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, Nd 3+ -containing glasses were made by melting with 50P 2 O 5 -(50 - x )BaO- x Nd 2 O 3 ( 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 Nd 3+ -containing glasses generally increased with Nd 2 O 3 concentration also resulting in shorter Nd 3+ -Nd 3+ distances. XRD supported the amorphous nature of the glasses, whereas the Raman spectra evolution was indicative of glass depolymerization being induced by Nd 3+ ions. Oxygen (1s) and phosphorus (2p) analysis by XPS for the glass with 4.0 mol % Nd 2 O 3 agreed with the increase in nonbridging oxygens relative to the undoped host. DSC results showed that the glass transition temperatures increased with Nd 3+ 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 Nd 2 O 3 content. A glass strengthening/tightening effect was then indicated to be induced by Nd 3+ with higher field strength compared to Ba 2+ ions. The UV-vis-NIR absorption by Nd 3+ ions increased consistently with Nd 3+ 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 Nd 3+ NIR emission intensity was highest for 1.0 mol % Nd 2 O 3 and decreased thereafter. The decay kinetics of the 4 F 3/2 emitting state in Nd 3+ 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 Nd 3+ concentrations.