Thermal, structural and optical properties of lithium niobate tellurite glass doped erbium and neodymium

A series of tellurite glasses of composition (70–x–y)TeO2–15Li2CO3–15Nb2O5–xEr2O3–yNd2O3 doped Er3+ and Nd3+, with x=0, 1.0 mol%; 0 ≤ y ≤1.0 mol% were prepared by using melt quenching technique. The glass phase and thermal behaviour were investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) while the structural and optical properties were investigated using Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible–near infrared spectroscopy (UV–VIS–NIR) and photoluminescence spectroscopy. The XRD spectra confirmed that the glass was amorphous as no sharp peaks were observed. The thermal parameters particularly the thermal stability is around 291.5°C and Hruby criterion of around 0.82 to 0.93. The structural properties of the glass represented by the FTIR spectrum indicate that as Nd2O3 content increases, the sharp infrared absorption peaks shifted from 474.7 cm-1 to 499.4 cm-1. These peaks are due to Nb–O, Te–O–Te and O–Te–O bond linkage bending vibration. For TeO4 trigonal bipyramid, the peak occurred at 676.5 cm-1 whereas for TeO3 trigonal pyramid, two infrared band peaks were observed at 787.5 cm-1 and 887.6 cm-1. The absorption peaks around 1382.7 cm-1 is due to the Te–O–Nb stretching vibration while peaks at 1635.5 cm-1 and 3411.7 cm-1 are due to the stretching vibrations of the hydroxyl group participating in the strong metal bonding as well as in the hydrogen bonding, respectively. The UV–VIS–NIR spectrum exhibits absorption peaks corresponding to transitions from both ground state of Erbium, 4I15/2 to the excited state of 4F7/2, 2H11/2, 4S3/2, 4I9/2, 2H9/2, 4I11/2, 4I3/2 and 4I13/2 and Neodymium, 4I9/2 to the excited state of 2G11/2, 2G9/2, 2G7/2, 4F9/2, 4I3/2 and 4I15/2. The up conversion was observed in the luminescence spectra by the red emission at around 633 nm which is due to the transition from 4F9/2 4I9/2. The down conversion was represented by the green emission at 497 nm due to transition from 2G9/2→4I9/2