Structural, luminescent properties and Sm3+ → Sm2+ reduction in H2 gas atmosphere of Tb3+/Sm3+ co-activated lithium sodium borate glasses

Abstract

This work presents the impact of lithium sodium borate (BLN) glasses with the molar composition $(69.25-x)$B${}_2$O${}_3+15$Li${}_2$O$+15$Na${}_2$O$+0.75$Tb${}_2$O${}_3+x$Sm${}_2$O${}_3$ ($x=0,0.75,1.0,1.25,1.5$, and 1.75 mol%) on the structure and optical properties. The amorphous nature of the fabricated glasses was confirmed by XRD patterns and SEM images. The glass samples reached a density from 2.38 to 2.61 g/cm${}^3$ and their molar volumes decreased from 27.20 to 26.67 cm${}^3$/mol with the increase of the Sm${}_2$O${}_3$ concentrations. FTIR and Raman spectra analysis confirmed the main vibrations located around 600, 1300, and 1600 cm⁻¹ assigned to the bending vibration B-O-B bridges, asymmetric stretching of BO${}_3$, and B–O${}^{-}$ stretching, respectively. Analysis of photoluminescence (PL) spectra indicated that Tb${}^{3+}$ shows emission from both ${}^5$D${}_3$ and ${}^5$D${}_4$ excited levels, and Sm${}^{3+}$ emits three bands at 563, 600, and 646 nm corresponding to the ${}^4$G${}_{5/2}{\to }^6$H${}_J$ ($J=5/2,7/2,$ and $9/2$) transitions. The BLN glasses co-doped with Tb${}^{3+}$/Sm${}^{3+}$ can act as color-tunable phosphors, exhibiting an overall emission change at different Sm${}^{3+}$ concentrations. The lifetime result of Tb${}^{3+}$ emission indicate energy transfer from Tb${}^{3+}$ to Sm${}^{3+}$ ions due to the dipole-quadrupole interaction. The CIE coordinates of these glasses vary from light-blue (0.2214, 0.3032) to the near white light (0.3501, 3403) regions by modifying Sm${}^{3+}$ concentrations. The reduction phenomenon from Sm${}^{3+}$ to Sm${}^{2+}$ ions in the BLN glasses was also observed when the prepared glasses were annealed in the 100% H${}_2$ gas atmosphere. The ratio of (Sm${}^{2+}$/(Sm${}^{3+}$+Sm${}^{2+}$)) emission intensity increased from 0 to 25% when the annealing period was changed from 1.0 to 20.0 h.