Effect of flux compounds on the luminescence properties of Eu³⁺ doped YBO₃ phosphors

To produce YBO₃:1%Eu³⁺ phosphor via solid state synthesis, the starting materials including yttrium acetate (Y(CH₃COO)₃·H₂O), boric acid (H₃BO₃), europium oxide (Eu₂O₃), lithium chlorides (LiCl), barium chlorides (BaCl₂) and calcium chlorides (CaCl₂) were purchased at the highest possible grade from Aldrich. In a typical synthesis of YBO₃:1%Eu³⁺ phosphor, specific amounts of yttrium acetate, boric acid and eu-ropium oxide were mixed in an alumina crucible, followed by heating in a tube furnace at 1100°C for 2 hours.

The crystal structures were analyzed by X-ray diffraction with CuKα radiation (λ = 1.54 Å). The morphology of the powders was observed by scanning electron microscope (JSM 6360) and field emission scanning electron microscope (JSM 6330F). Also, X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific, UK) and photolumi-nescence excitation and emission (PL, FTP Felix 32, Japan) were employed for characterization of synthesized phosphors.

Fig.1 shows the XRD spectra of the synthesized solid state YBO₃:Eu³⁺ phosphors. The figure confirms that these materials are well crystallized with a hexagonal crystal structure (JCPDS# 16277). Obviously, in the phosphors synthesized with only 2 % flux compounds, no remarkable differences can be observed compared to those synthesized without any additives. By contrast, when the amount of flux compounds reaches 5 % or 10 %, some extra peaks have emerged in the XRD spectra. For instance, when LiCl has been consumed, an extra peak at approximately 25° has arrived, while additional peaks have been generated at 25.5° and 36° for BaCl₂ and 30° for CaCl₂.

For the phosphors synthesized with small amounts of flux compounds, ICP analysis was employed to judge about the presence or absence of remaining flux compounds after solid state synthesis at 1100 °C. The results of ICP (not shown) confirmed the remaining of Li, Ba and Ca after solid state synthesis. Also, as the boiling points of LiCl, BaCl2 and CaCl₂ are 1382 °C, 1560 °C and 1935 °C, respectively, it is evident that the employed calcination temperature is not sufficient for evacuating the flux compounds.

Fig. 2 shows the SEM microstructure of YBO₃:Eu³⁺ phosphors, without and with different concentrations of flux compounds. It is easily observed that in the presence of flux compounds, the particle size of phosphors increases significantly.

It can be found that the obtained average particle size is about 1.2 μm when no flux is used in the solid state procedure. Instead, in the presence of flux compounds, the size of particles is in the range of 1.9 μm to 4.8 μm, depending on the type and quantity of employed flux compounds. Hence, it is concluded that regardless of flux type, the addition of flux compounds improves the sintering and the crystal growth. According to Fig. 3, it is seen that LiCl has the strongest effect on the growth of phosphor particles, increasing the particle size from 1.2 μm to 4.8 μm. Conversely, CaCl₂ has a relatively weak influence on the growth of particles. The growth rate of particles in the presence of flux compounds can be estimated from the following equation: (1)dφ/dt=Aexp−ΔEKT$$d\varphi /dt = {A_{exp }}\frac{{ - \Delta E}}{{KT}}$$

In this equation, dφ/dt, A, K, ΔE and T rep-resent particle growth rate, flux related constant, Boltzmann constant, activation energy and synthesizing temperature, respectively [18]. Clearly, the particle growth in the presence of flux compounds depends mainly on the synthesis temperature and the activation energy. The above equation reveals that an increase of temperature and decrease of activation energy, accelerate the crystal growth. As the melting points of LiCl, BaCl₂ and CaCl₂ are about 610 °C, 962 °C and 775 °C, respectively, at the solid state synthesis temperature (T = 1100 °C), all the flux compounds are molten. So, it can be assumed that for the nuclei in themixed oxide system, the activation energy can be written as [18]: (2)ΔE=NGv+σλ$$\Delta E = N\left( {Gv + \sigma } \right)\lambda$$(3)ΔE∼Nσλ$$\Delta E\ {\sim}\ N\sigma \lambda$$

This implies that the change in free energy depends directly on the number of nuclei (N), surface energy (σ) and the volume of nuclei (λ).

It can be found that flux composition affects the surface energy and so activation energy, significantly. Also, referring to the presented comparison of growth rates, LiCl provides the lowest surface energy.

Fig. 4 shows the photoluminescence excitation (PLE) and emission spectra of synthesized solid state YBO₃:Eu³⁺ phosphors under λ_em = 592 nm and λ_exc = 240 nm, respectively. It has already been proved that the broad band in the range of 200 nm to 260 nm belongs to the charge transfer band (CTB) of Eu³⁺−O²⁻, since an electron trans-fers from the oxygen orbit (2p⁶) to the empty states of Eu³⁺ (4f) [8]. Also, in the PL emission spectra, the observed emission peaks in the wavelengths larger than 575 nm are associated with the transitions from the excited ⁵D₀ level to ⁷F_j (J = 1, 2, 3, 4) levels of Eu³⁺ activators [19]. The strong band observed at 592 nm is related to the ⁵D₀ → ⁷F₁ magnetic dipole transition of trivalent Eu ions. In YBO₃ host lattice with a hexagonal crystal structure, since Eu³⁺ ions are substituted into Y³⁺ locations similar to Y³⁺ ions, Eu³⁺ ions are also surrounded by BO3 groups and possess a symmetry center implying a strong ⁵D₀ → ⁷F₁ transition. Also, the bands at approximately 611 nm and 627 nm are attributed to the ⁵D₀ → ⁷F₂ electric dipole transitions [20]. In the PL measurements of the phosphors synthesized with flux compounds, with an increase in the amount of fluxes, no main change in the shape or position of peaks could be observed, except the intensity of the peaks. Also, the addition of up to 2 wt.% of LiCl, BaCl₂ and CaCl₂ results in enhancement of the photoluminescence intensities. But the use of larger quantities of fluxes suppresses emission of YBO₃:Eu³⁺ phosphors. The improvement of PL intensity in the presence of low amounts of flux compounds may be attributed to the improved crystallinity as well as the enlarged grain size, explained elsewhere. On the other hand, as it was discussed for the XRD spectra, the use of relatively large amounts of flux compounds results in the formation of some impurities in the crystal structure of YBO₃ phosphors. This phenomenon plays a vital role in suppressing the photoluminescence intensities. Noteworthy, the intensity ratio of the ⁵D₀ → ⁷F₁ transition to that of ⁵D₀ → ⁷F₂ transition depends strongly on the local symmetry of Eu³⁺ ions. In short, when Eu³⁺ ions occupy the inversion center sites, this ratio will be larger and the produced phosphor looks more reddish.