Luminescence properties of dual valence Eu doped nano-crystalline BaF _2 embedded glass-ceramics and observation of Eu ~(2+) → Eu ~(3+) energy transfer

摘要

Europium doped glass-ceramics containing BaF _2 nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF _2 nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm ~(-1) has been demonstrated from the emission spectra of Eu ~(3+) ions showing the transitions from upper excitation states 5DJ (J=1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu ~(3+)) ions to divalent (Eu ~(2+)) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu ~(2+) to Eu ~(3+) ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu ~(3+) to Eu ~(2+) under normal air atmospheric condition has been observed in a BaF _2 containing oxyfluoride glassceramics system.