Correlation between luminescence and crystallization characteristics of Dy³⁺ doped P₂O₅–BaO–SeO₂ glasses for white LED applications

Ternary phosphate host glass with the basic composition (P₂O₅ 50%, BaO 40%, SeO₂ 10%) with varying Dy₂O₃ dopants (0.125, 0.25, 0.5, 1%) were prepared by the melting and annealing technique. The prepared glasses were collectively investigated by optical, FTIR and photoluminescence spectral measurements. The work was supplemented through thermal expansion and differential thermal measurements of the glasses to be able to conduct controlled thermal heat-treatment of the parent samples and converting them to their glass–ceramics derivatives. X-ray diffraction and scanning electron microscopic studies of the glass–ceramics were conducted to identify the separated crystalline phases during thermal heat-treatment and their morphological textures. The optical spectra of the dysprosium ions-doped glasses reveal 11 absorption bands in two rows extending from 350 to 476 nm and from 754 to 1700 nm which are assumed to be originating from the excitations of the ground state ⁶H_15/2 to different transitions. The photoluminescence spectra show two characteristic emission bands in the blue and yellow region beside one faint band. The two characteristic emission bands are assigned to ⁴F_9/2 → ⁶H_13/2 and ⁶F_9/2 → ⁶H_15/2 transitions. The CIE chromaticity coordinates have been evaluated from the emission spectra to understand the suitability of the glasses for white light emitting diode. FTIR absorption spectra of the glasses and glass–ceramics show composite broad bands within the mid IR region and the deconvoluted spectra indicate the appearance of characteristic bands due to phosphate groups besides the sharing of Se–O vibrations. The thermal expansion data show the decrease of softening temperature with the introduction of 1% Dy₂O₃ in conformity of the presence of RE³⁺ ions in modifying positions. The DTA and X-ray data have been correlated with the housing of Dy³⁺ ions as modifiers within the glass structure and their action on the depolymerization of phosphate network which is assumed to promote the increase of barium selenium oxide crystalline phase during the conversion to glass–ceramic derivatives and the simultaneous limited decrease of the second main barium phosphate crystalline phase.