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

Abstract

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.