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

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 Eu(3+) → Eu(2+) was reported when Eu(3+) 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 Eu(2+) ions. The defect complexes of [(Eu(3+)Cs)(••)-2VCs'] or [(Eu(3+)Cs)(••)-Oi″] induced by the substitution of Eu(3+) on Cs(+) were suggested in the lattices. Eu(2+) ions could be regarded as Eu(3+) ions combining with the released electrons from defects Oi″ or VCs' in close vicinity of Eu(3+) (Eu(3+) + e); the electrons cannot enter the atom track of Eu(2+) presenting luminescence of Eu(2+) ions. The results indicate that several defect traps can be attributed to the abnormal reduction mechanism of Eu(3+) to Eu(2+) ions in a matrix.