Luminescence enhancement by the reduction–oxidation synthesis in monoclinic RE2O3 (RE=Eu, Gd) phosphors containing Eu3+ activator

A novel synthesis was developed for enhanced luminescence in sesquioxide phosphors containing Eu3+ activator. It consisted of two annealing steps: reduction under vacuum with gaseous H2 at 10 Torr and 1300 °C and re-oxidation at 300–1500 °C in air. The integrated luminescence intensity of the monoclinic Eu2O3 phosphor was enhanced ca. 21 times by this method compared with conventional processing. The photoluminescence (PL) intensity was maximized at re-oxidation temperatures of 500–1100 °C. The PL characteristics of monoclinic Eu2O3 and Gd2O3:0.06Eu samples were compared with a commercial cubic Y2O3:Eu phosphor. The evolution of physical characteristics during the two-step annealing was studied by Raman spectroscopy, XPS, XRD, PL decay analysis, and SEM. PL decay lifetime increased proportionally to the PL intensity over the range 0.5–100 μs. Additional vibrational modes appeared at 490, 497, and 512 cm−1 after the two-step annealing. The increase in PL intensity was ascribed to the formation of excess oxygen vacancies and their redistribution during annealing. Resonance crossovers between the charge transfer state and the emitting 5DJ states are discussed in relation to reported luminescence saturation mechanisms for oxysulfides Ln2O2S:Eu3+ (Ln=Y, La).