Synthesis, Luminescence Properties And Thermal Stability Of BaMgAl₁₀O₁₇: Eu～(2+) Blue Phosphors

The structure of BaMgAl₁₀O₁₇, orβ-alumina, consists of two spinel blocks (MgAl10O16) separated by one mirror plane (BaO). When Eu²⁺ is substituted into the host lattice, it can have been three possible locations: Beevers–Ross (BR), anti-Beevers-Ross (a-BR), and mid-oxygen (mO) sites in the mirror plane. On the mechanism of the deterioration, much research had been conducted. In the literature, luminance deterioration is usually attributed to the oxidation of BaMgAl₁₀O₁₇: Eu²⁺ phosphor and the changes of the environments surrounding the europium ion。They indicated that by oxidation, BaMgAl₁₀O₁₇: Eu²⁺ was converted into a mixture of two compounds BaMgAl₁₀O₁₇ and Eu (III) MgAl11O19. This deactivated the Eu²⁺ luminescent center and decreased the Eu²⁺ emission. The important thing that should be noted is that the Eu³⁺ no longer prefers to substitute for the Ba²⁺, as the Eu²⁺ did. Instead it would prefer to substitute for an aluminum ion in the Al (2) positions, that is, a tetrahedral site. The computer simulation results show that Eu²⁺ prefers to occupy the BR and mO site in the cation layer while Eu³⁺ prefer the Al (2) tetrahedral position in the spinel block. Recently, some other opinions were demonstrated, such as generation of a lattice vacancy, migration of Eu²⁺ to the spinel block, intercalating water molecules into the host lattice, and the transfer of the electron from the divalent europium ions to adsorbed oxygen ions.In this paper, the blue phosphors were synthesized by sol-gel process, the effect of the firing temperature and the Eu²⁺ concentration on the distribution of Eu²⁺ among different sites was firstly investigated through the changes of excitation spectra with changing fired temperature and Eu²⁺ concentration. There are two wide bands one located at short wavelength peaking at 250 nm and the other at long wavelength peaking at 310 nm. As annealing temperature increases, the overall intensity increases, as expected. In addition, long wavelength excitation generates more emission than short wavelength excitation at lower annealing temperatures. At 1100°C, the relative emission intensities in the two regions are about the same, and further increasing the annealing temperature to1300°C, the short wavelength excitation becomes more efficient. Since the Eu³⁺ ions occupy Al (2) sites in the middle of the spinel block and the Eu²⁺ ions occupy the BR site in the mirror plane, it is necessary for europium to migrate from the Al (2) site to the BR and mO site in the process of the reduction from Eu³⁺ to Eu²⁺. However, there is about 5 ? distances between both sites, to migrate, europium must obtain certain energy. When the annealing temperature is lower than 900°C, due to the Eu ³⁺ occupying the Al (2) site in the middle of the spinel block, it could not obtain enough energy to migrate to the mirror plane at relatively low temperature and the Eu³⁺ is not reduced to Eu²⁺, therefore we could not observe the blue emission. More and more Eu³⁺ obtaining enough energy migrate to the cation layer with increasing the temperature and at the same time the Eu³⁺ is reduced to Eu²⁺, therefore, the blue emission is observed and becomes stronger with increasing temperatures. The bigger is the crystal effect of site occupied by Eu smaller, the bigger the energy needed to accommodate the Eu²⁺. (We think that Eu²⁺ located at BR and mO site have weaker crystal effect, and that locate at aa-BR site have stronger crystal effect.). So the Eu²⁺ ions occupy the BR and mO sites first at relatively low annealing temperatures. Therefore, the BR and mO site is the most energetically favorable site for Eu²⁺. More and more Eu³⁺ released from the A (2) site to the Ba site and reduced to Eu²⁺ with increasing annealing temperature in a reducing atmosphere and at the same time more and more Eu²⁺ migrating from Al (2) sites or BR and mO sites would occupy the a-BR sites other than BR and mO sites. The distribution of Eu²⁺ is not random. So, when the annealing temperature is low, the Eu²⁺ mainly occupy the BR and mO site and the Eu²⁺ tend to occupy both BR and mO sites and a-BR sites with increased temperature. The two wide bands originate from the transitions from the 4f7 ground state of the Eu²⁺ to the T2g and Eg excited states of the 4f65d configuration. Relative rate of transition to T2g and Eg should be different because the environment surrounding the Eu²⁺ is different. The excitation spectra demonstrate that the BR and mO site excitation is more effective in the long wavelength peaking at 310 nm, but the a-BR site is more effective in the short wavelength peaking at 250 nm. As previously indicated, the ratio of a-BR site to BR and mO site is increased with increasing the annealing temperature. Therefore, the long wavelength excitation at 310 nm is more effective at lower annealing temperature, but the short wavelength excitation is more effective at higher temperature (see fig. 4). The experimental results are well consistent with our proposal. Based on our results, we proposed a model of the reducing process: (1) the Eu³⁺ located in the Al (2) sites absorb some energy, (2) it migrate from the A l (2) site to the BR and mO site, (3) Eu³⁺→Eu²⁺, (4) the Eu²⁺ at the BR and mO site absorb more energy, (5) the migration of Eu²⁺ from the BR and mO site to the a-BR site. Finally the Eu²⁺ ions occupy more than one site.Therefore, the a-BR/BR and mO ratio increased with increasing annealing temperature and Eu concentration. The Eu²⁺ located at the a-BR site is more stable than that located in the BR and mO site. Therefore, the thermal stability of blue phosphors could be improved through increasing annealing temperature and Eu concentration. Additionally, the BAM: Eu²⁺ was coated with SiO2. It is worth noting that the BAM phosphor with SiO2 coating shows an increase in the luminescence intensity under 254 nm excitation. However, no obvious increase in the luminescence intensity is observed for the147 nm excitation. After coating, the changes of emission intensity result from two competitive effects: on one hand, the coating weakens the reflectivity and increases the number of photons that transmit into the phosphor. On the other hand, the coating also absorbs photons, leading to the decrease of the numbers of photons that get to the phosphors. In our experiment, the former is dominant for 254 nm excitation, but the latter is dominant for 147 nm excitation. In conclusion, as the wavelength of excitation light becomes shorter, the absorption of coating and phosphors becomes larger and then the penetration depth of light becomes short. Therefore, the SiO2 coating provides an increase in peak intensity under 254 nm excitation while no obvious increase is observed under 147 nm excitation.The SiO2-coated and uncoated-BAM were annealed simultaneously in air. We found that luminescent intensity of BAM after coating is higher than that uncoated. The SiO2 coatings protect the phosphor surface by prohibiting further oxidation and migration of Eu²⁺ during the firing process. This enhances the thermal stability of phosphors, which is critical in PDP and FL manufacturin...