Investigations on the spectroscopic properties and local structure of Eu3+ ions in zinc telluro-fluoroborate glasses for red laser applications

Abstract

The strong red emitting Eu3+ doped Zinc telluro-fluoroborate glasses with the chemical composition (30–x)B2O3+30TeO2+16ZnO+10ZnF2+7CaF2+7BaF2+xEu2O3 (where x = 0.1, 0.25, 0.5, 1.0, 2.0 and 3.0 in wt%) have been prepared following the melt-quenching technique and their structural, spectroscopic behavior were characterized. The local structure and vibronic spectral analysis were performed in order to correlate them with the local field environment of Eu3+ ions in the prepared Zinc telluro-fluoroborate glasses on the basis of FTIR, Raman, Phonon sideband spectroscopy. Fundamental stretching units of Te–O bond in TeO3/TeO4 units, stretching of B–O bond in BO3/BO4 units and bending vibrations of Te–O–Te/B–O–B bonds were identified through FTIR and Raman spectral analysis. The Nephelauxetic intensity ratios (β) and bonding parameter (δ) values obtained from the absorption bands reveals the covalent nature of the Eu–O bond in the prepared glasses. Phonon sidebands appear around 505, 732 and 999 cm−1 results from the combined bending vibrations of Zn−O−Te and Te−O−Te in borate network, combined stretching vibrations of TeO4 trigonal bipyramidal (tbp) units with the formation of vitreous B2O3 due to the bending vibration of B–O–B linkages, and B−O bond stretching in BO4 units, stretching modes of terminal oxygens (B–O−) belong to the BO2O metaborate triangles respectively. The JO intensity parameters (Ωλ) and the asymmetric intensity ratio (R) values calculated from the emission spectra have been used to explore the nature of the bonding and asymmetry around the Eu-ligand field environment. The increasing Ω2 intensity parameters and R values confirms the higher asymmetry around the Eu3+ ions and covalency of Eu–O. The JO intensity parameters were used to estimate the radiative properties such as transition probability (A), stimulated emission cross-section (σPE), branching ratio values (β) and radiative lifetimes (τR) for the observed emission transitions. The excited state lifetime for the 5D0 level of the Eu3+ ions exhibit single exponential behavior for all the titled glasses and found to increase with the increase in Eu3+ ions concentration due to the higher asymmetry around Eu3+ ions. Further, emission intensities were characterized using CIE chromaticity diagram and the (x,y) chromaticity coordinates (0.63, 0.36) suggests the pure intense red emission obtained from the prepared glasses useful for laser applications at 616 nm.