Abnormal Reduction, Eu3+ → Eu2+, and Defect Centers in Eu3+-Doped Pollucite, CsAlSi2O6, Prepared in an Oxidizing Atmosphere

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Abstract
Eu-doped pollucite CsAlSi2O6 was synthesized by the sol–gel method and heated in an air atmosphere. The crystal structure and the microstructure of the phosphors were investigated by X-ray powder diffraction and SEM images, respectively. The photoluminescence spectra and temperature dependent decay curves were measured. An abnormal reduction phenomenon of Eu3+ → Eu2+ was reported when Eu3+ ions were doped in alkaline metal cation sites in CsAlSi2O6 prepared in an oxidizing atmosphere. The abnormal mechanism was discussed on the basis of the charge compensation model and a rigid three-dimensional framework structure of CsAlSi2O6. The luminescence color centers were investigated by luminescence decay lifetimes and thermal stabilities of Eu2+ ions. The defect complexes of [(Eu3+Cs)••–2VCs′] or [(Eu3+Cs)••–Oi″] induced by the substitution of Eu3+ on Cs+ were suggested in the lattices. Eu2+ ions could be regarded as Eu3+ ions combining with the released electrons from defects Oi″ or VCs′ in close vicinity of Eu3+ (Eu3+ + e); the electrons cannot enter the atom track of Eu2+ presenting luminescence of Eu2+ ions. The results indicate that several defect traps can be attributed to the abnormal reduction mechanism of Eu3+ to Eu2+ ions in a matrix.

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