Firing temperature dependence of Eu diffusion in Eu-Y2O2S studied by 89Y MAS NMR

Abstract

⁸⁹ Y MAS NMR (magic angle spinning nuclear magnetic resonance) and powder X-ray diffraction were applied in order to elucidate the crystal growth process, Eu doping process, and Eu distribution in a red phosphor, Eu-doped Y₂O₂S (Eu-Y₂O₂S), fired at varying temperatures, as well as the correlation between such material properties and the phosphor quality. The spin–lattice relaxation time (T₁) was measured for the main peak at 292 ppm due to Y atoms located far from the paramagnetic Eu and for the satellite peaks due to three types of Y¹ atoms [here, Y¹ is the Y atom μ-bridged by O and/or S atoms to an Eu atom, such as Y¹(–O–,–S–)Eu]. The T₁ values for Y¹, T₁(Y¹), were independent of the firing temperature from 900 to 1200°C and depend on the type of μ-bridge, indicating that T₁(Y¹) is not influenced by crystal characteristics such as crystallite size and degree of lattice disorder, but is influenced by the interatomic distance between Y¹ and the doped Eu. The T₁ values of the main peak, T₁(Yⁿ), were influenced by the Eu distribution. Comparing T₁(Yⁿ) to parameters such as the chromaticity coordinates, brightness and intensity of emission, it was found that the substitution of Eu at the Y site in Y₂O₂S and the random distribution of Eu are accelerated with the elevation of the firing temperature from 900 to 1200°C. The amount of Eu substituted at random into the Y₂O₂S crystal was evaluated as the effective Eu concentration for monitoring the properties of the phosphor. The interrelation between the brightness, the line-width and T₁(Yⁿ) in ⁸⁹Y NMR, the amount of effective Eu, and the crystallinity of Eu-Y₂O₂S was elucidated.