Synthesis and photoluminescence properties of Ba₃Al₂O₆:Eu³⁺ red phosphor

Abstract

Eu³⁺-doped Ba₃Al₂O₆ red phosphors were synthesized by high-temperature solid-state reaction. The crystal structure, morphology and luminescence properties were investigated by XRD, TEM and photoluminescence spectroscopy. The effect of calcination temperature, Eu³⁺ concentration as well as charge compensators on the luminescence properties was also investigated. Characteristic orange-red emission at 589 nm was detected under 394 nm excitation. The best performance could be derived for the sample calcined at 1450 °C. With the increase of Eu³⁺ concentration, the emission intensities increased gradually up to the Eu³⁺ concentration of 12 %. The emission intensities could be enhanced with Li⁺, Na⁺ and K⁺ introduced as charge compensators. The results showed that Ba₃Al₂O₆:Eu³⁺ phosphors with charge compensator had potential application in the fields of near-UV-excited WLEDs.

Introduction

Recently, white light-emitting diodes (WLEDs) have attracted much attentions due to its compactness, high efficiency, long operational lifetime, energy saving and environment protection [1–4]. However, the commercial WLEDs fabricated using yellow-emitting YAG:Ce³⁺ phosphors with blue LED chips have a notable deficiency because of the scarcity of red emission, restricting their applications in some important fields, especially in lighting markets [5]. Nowadays, near-UV LED chips coated with red, green and blue phosphors are considered as a potential system to produce better properties in emission efficiency, colour temperature and colour rendering index [6, 7]. Comparing with the blue and green phosphors, the commonly used red phosphors for WLEDs show lower chemical stabilities and fluorescence efficiencies. Therefore, stable and high-efficient red phosphors suitable for near-UV chips are necessary for the development of WLEDs.

Among all the host materials used in WLEDs, rare earth-doped Alkaline earth aluminates M₃Al₂O₆ (M = Ca, Sr, Ba) possess many advantages, such as high chemical stability, high luminescence intensity, high quantum efficiency, nontoxic, low cost and so on [8]. The structure of M₃Al₂O₆ belongs to the cubic system with space group Pa3 [9–11]. M has six independent crystallographic sites in M₃Al₂O₆ lattices [12]. Recently, it is reported that Eu³⁺- or Dy³⁺-doped Ca₃Al₂O₆ and Eu²⁺-doped Sr₃Al₂O₆ can be used as promising phosphors for fabrication of warm WLED [13–15]. In addition, Ca₃Al₂O₆ and Sr₃Al₂O₆ are popular hosts in M₃Al₂O₆ system and the luminescence properties of rare earth ions doped in these two hosts have been investigated [8, 16, 17]. But to our best knowledge, there are few reports on the photoluminescence properties of rare earth-doped Ba₃Al₂O₆. In this work, the Eu³⁺-doped Ba₃Al₂O₆ phosphors are prepared by the method of high-temperature solid-state reaction. The luminescence properties of Ba₃Al₂O₆:Eu³⁺ are investigated, and the effect of calcinations temperature, Eu³⁺ concentration as well as charge compensations on the luminescence properties is also discussed.

Experiment

Ba₃Al₂O₆:Eu³⁺ phosphor was synthesized by high-temperature solid-state method using Eu₂O₃ (99.99 %), BaCO₃ (AR), Al₂O₃ (AR), Li₂CO₃ (AR), Na₂CO₃ (AR) and K₂CO₃ (AR) as reagents. Two types of samples were prepared: Ba₃Al₂O₆:xEu³⁺(x = 0.02, 0.04,…, 0.16), Ba₃Al₂O₆:0.06Eu³⁺, 0.06Z (Z = Li⁺, Na⁺, K⁺). According to the formula above, stoichiometric amounts of starting materials were weighed and then mixed thoroughly in the agate mortar. The mixture was calcined at different temperatures (1300, 1350, 1400 and 1450 °C) in a muffle furnace in air atmosphere for about 6 h. After cooling down to room temperature, white powder samples were obtained and then reground.

X-ray diffraction patterns of the samples in the range of 10° ≤ 2θ ≤ 90° were recorded on a Bruker D8 Advance X-ray diffractometer with Cu K _α radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) micrographs were taken with a Tecnai G2 F20 field-emission transmission electron microscope. The excitation and emission spectra of the samples were detected by Hitachi F-7000 fluorescence spectrometer. The light source was xenon lamp and resolution was 0.5 nm. The light emitted from the Xe lamp entered the excitation side monochromator and then incident on the sample. Luminescence signal emitted from the sample directed into the emission side monochromator and was condensed onto the photomultiplier tube by concave mirror, whereby its intensity was measured. To ensure the identical measurement conditions of photoluminescence spectra for the different samples, the slit width, scan speed and high voltage of photomultiplier tube were kept unchanged for each measurement. All measurements were carried out at room temperature and the spectra had been corrected.

Results and discussion

Photoluminescence characteristics

The photoluminescence spectra of Ba₃Al₂O₆:0.06Eu³⁺ calcined at 1450 °C are presented in Fig. 1. In the excitation spectrum found in Fig. 1a, there are broadband centred at about 270 nm associated with the charge transfer (CT) transition from 2p orbital of O²⁻ ions to 4f orbital of Eu³⁺ ions and narrower bands arising from intraconfigurational 4f–4f transitions (assignment is given in Figure). The emission spectrum of Ba₃Al₂O₆:0.06Eu³⁺ sample under 394 nm excitation (Fig. 1b) consists of two emission bands with peaks at 589 and 612 nm, corresponding to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions. The most intense peak located at 589 nm confirms that Eu³⁺ ion occupies the lattice site with inversion centre of symmetry [18, 19].

Figure 1

The photoluminescence spectra of Ba₃Al₂O₆:0.06Eu³⁺ calcined at 1450 °C. a Excitation spectrum measured while detecting the emission line at 589 nm; b emission spectrum measured under 394 nm excitation

The effect of calcination temperature on the photoluminescence properties of Ba₃Al₂O₆:0.06Eu³⁺

The XRD patterns of Ba₃Al₂O₆:0.06Eu³⁺ calcined at four different temperatures are shown in Fig. 2. It can be seen from the figure that all the diffraction peaks of samples show very similar profiles and they are in agreement with that of the standard card (JCPDS No. 25-0075), which means that cubic phase Ba₃Al₂O₆ is predominant in the samples. Some additional peaks at about 22.5° and 36.2° are observed for the samples with calcination temperature 1300 and 1350 °C in Fig. 2. These additional peaks maybe attributed to the diffraction of BaCO₃ (JCPDS No.52-1528). The decomposition temperature of BaCO₃ is 1300 °C, so it may not be decomposed completely at 1300 and 1350 °C because of the temperature fluctuation in the furnace. The intensities of diffraction peaks increase when the calcination temperature increases from 1300 to 1450 °C, which indicates that higher crystalline is obtained at higher calcination temperature.

Figure 2

The XRD patterns of Ba₃Al₂O₆:0.06Eu³⁺ calcinated under different temperatures

Figure 3 shows the TEM images of Ba₃Al₂O₆:0.06Eu³⁺ calcined at different temperatures. It is found that the morphology of the sample depends on the calcination temperature heavily. The particles of sample calcined at 1300 °C are irregular with sizes in the range of 40–60 nm and aggregated evidently, as shown in Fig. 3a and b. The high-resolution TEM (HRTEM) image exposes the crystalline nature of the sample. In Fig. 3c, lattice fringes can be observed and the interplanar spacing is 0.375 nm, indexed to be (331) planes. However, most of the fringes cannot be observed clearly in the detected region, which means that amorphous phase is predominant in this sample and the crystallinity is poor. With the increase of calcinations temperature, whole of the samples are converted into nanorods and the nanorods are straight in nature. What is more, lattice fringes become clearer. The formation of these varying morphologies is not yet well understood. As can be seen from Fig. 3d and e, the nanorods calcined at 1350 °C have the range of diameter 40–80 nm and length in the range from 2 to 8μm. HRTEM image shows very clear lattice fringes and the amorphous phase has disappeared (see Fig. 3f). The obvious lattice fringes confirm the high crystallinity of the sample calcined at 1350 °C. The interplanar spacing of lattice fringes is 0.317 nm, ascribed to (511) planes of Ba₃Al₂O₆. However, a part of lattice sites are not aligned or go right through the entire array, indicating the presence of defects in the crystal. For the samples calcined at 1400 and 1450 °C, the length of nanorods decreases to about 2–6μm and the diameter increases to 50–110 nm (see Fig. 3g, h, j, k). Further, HRTEM analysis reveals that the lattice fringes with the interplanar spacing of 0.333 and 0.441 nm are present and assigned to the (442) and (321) planes for the sample calcined at 1400 °C (see Fig. 3i). The orientations in different directions show the polycrystalline nature of sample [20]. While, as for the sample calcined at 1450 °C, only lattice fringes of (422) planes can be observed (see Fig. 3l), which means the sample is of single-crystalline nature. Therefore, it is concluded that the crystallinity increases with the increase of calcinations temperature, which is consistent with the XRD results. In general, all experimental results discussed above confirm that morphology and crystallinity of Ba₃Al₂O₆ are heavily affected by the calcination temperature.

Figure 3

TEM and HRTEM micrographs of Ba₃Al₂O₆:0.06Eu³⁺ calcined at 1300 °C (a, b, c), 1350 °C (d, e, f), 1400 °C (g, h, i) and 1450 °C (j, k, l)

Figure 4 shows the emission spectra excited at 394 nm for Ba₃Al₂O₆:0.06Eu³⁺ calcined at different temperatures. The emission intensity increases with the increase of calcination temperature. The reason may be due to the better crystallinity at higher calcination temperature, just as discussed above.

Figure 4

The emission spectra excited by 394 nm for the samples calcined at different temperatures

The effect of Eu³⁺ concentration on the photoluminescence properties of Ba₃Al₂O₆:Eu³⁺ samples

The XRD patterns of Ba₃Al₂O₆ samples doped with different Eu³⁺ concentrations are presented in Fig. 5. It is found that all the diffraction peaks of samples are in agreement with that of the standard card (JCPDS No. 25-0075) when the Eu³⁺ concentration (x) is lower than 0.04. Then the impurity phase BaAl₂O₄ (JCPDS No. 17-0306) appears when x exceeds 0.04 and the amounts of BaAl₂O₄ increase with the increase of Eu³⁺ concentration. Similar results are observed in rare earth-doped Ca₃Al₂O₆ phosphors [12]. This indicates that Eu³⁺ concentration has strong influence on the phase of synthesized Ba₃Al₂O₆ samples. Since the ionic radius of Eu³⁺ (0.095 nm) is smaller than that of Ba²⁺ (0.136 nm), the structure of Ba₃Al₂O₆ will be distorted when Eu³⁺ ions occupy the Ba²⁺ sites. According to the ratio of raw materials of Ba₃Al₂O₆:xEu, if Eu³⁺ replaces Ba²⁺, it will lead to an excess of BaO and Al₂O₃ and the Al₂O₃ will react with Ba₃Al₂O₆ to form BaAl₂O₄ [16, 21]. Usually, the diffraction peaks will move to the larger angles with increasing Eu³⁺ concentration when Ba²⁺ are replaced by the Eu³⁺ ions with smaller ionic radius. However, the diffraction peaks of samples move very slightly to the low-angle side with increase of Eu³⁺ concentration, demonstrating the increase of lattice constant in the samples [22, 23]. The reason may be that some Eu³⁺ ions are incorporated into the interstitial sites of crystal.

Figure 5

The XRD patterns of different Eu³⁺ concentration-doped Ba₃Al₂O₆:Eu³⁺ samples calcined at 1450 °C

Figure 6 presents emission spectra of different Eu³⁺ concentration-doped Ba₃Al₂O₆ samples calcined at 1450 °C under 394 nm excitation. It can be seen that photoluminescence intensity of ⁵D₀ → ⁷F₁ transition (589 nm) grows with increasing Eu³⁺ concentration up to x = 0.12. Further increase of Eu³⁺ concentration leads to intensity reduction because of concentration quenching. This quenching process is often attributed to energy migration among the Eu³⁺ ions. As the Eu³⁺ concentration increases, the distance between Eu³⁺ ions become shorter which may lead to growing energy migration between activator centres and thus enhanced probability of reaching a luminescence-killing defect [24, 25]. This optimal concentration of Eu³⁺ ions in Ba₃Al₂O₆ samples is lower than those in other aluminate hosts, such as YAG and SrAl₂O₄ [26, 27]. Eu³⁺ ion is known as a sensitive probe for the site symmetry. The red emission ⁵D₀ → ⁷F₂ transition is highly sensitive to the environment symmetry and a larger probability of this transition will be increased due to a decrease in symmetry. However, ⁵D₀ → ⁷F₁ transition hardly varies with the crystal field strength around Eu³⁺ ions and can be taken as a Ref. [28]. Therefore, the ratio of emission intensity of ⁵D₀ → ⁷F₂ transition to that of ⁵D₀ → ⁷F₁ transition, known as asymmetry factor R, is used to be a criterion for the site symmetries of Eu³⁺ ions. Generally, R is <1.0 for symmetric and >1.0 for noncentrosymmetric surroundings [29]. Figure 7 shows the dependence of asymmetry factor R on the Eu³⁺ concentration under 394 nm excitation wavelength in Ba₃Al₂O₆ samples. The R value is <1.0 and decreases with increase of Eu³⁺ concentration when x is lower than 0.12, which confirms that the Eu³⁺ ions are located in a symmetric environment, while the R values show an increasing tendency when Eu³⁺ concentration x exceeds 0.12, which means the decrease of local symmetry of Eu³⁺ ions. The reason is possible that the ion radius of Eu³⁺ is much smaller than that of Ba²⁺ ion. When Eu³⁺ ions enter the host lattice and substitute Ba²⁺ ions, barium vacancies are produced in order to keep charge balance in the materials, ${\text{Eu}}_{2} {\text{O}}_{3} \mathop{\longrightarrow}\limits{{{\text{Ba}}_{3} {\text{Al}}_{2} {\text{O}}_{6} }}2{\text{Eu}}_{\text{Ba}}^{.} + {\text{V}}_{\text{Ba}}^{{\prime \prime }} + 3{\text{O}}_{0}$. These defects will lead to the distortion of local environment symmetries of Eu³⁺ ions [30]. With the increase of Eu³⁺ concentration, more barium vacancies are induced, leading to lowering the symmetry of the surrounding Eu³⁺ ions [31]. For the sample in which Eu³⁺ concentration x = 0.16, the R value increases abruptly and is found to be 1.06, so the Eu³⁺ ions located in asymmetric environment are predominant. We think that impurity phase BaAl₂O₄ makes a great contribution to the R value. With the increase of Eu³⁺ concentration, the impurity phase cannot be negligible and Eu³⁺ ions may incorporate into BaAl₂O₄ host, so the real Eu³⁺ concentration doped into Ba₃Al₂O₆ samples maybe lower than that expected. On the other hand, the red emission of ⁵D₀ → ⁷F₂ transition is predominant in the Eu³⁺-doped BaAl₂O₄ phosphors because the Eu³⁺ ions occupy the barium sites with low symmetry [28]. Therefore, emission intensity of ⁵D₀ → ⁷F₁ transition (589 nm) decreases and that of ⁵D₀ → ⁷F₂ transition (612 nm) increases in high Eu³⁺ concentration-doped samples, and then intensity ratio of ⁵D₀ → ⁷F₂ transition to that of ⁵D₀ → ⁷F₁ transition increases according to the analysis above. This assumption can be proved by the emission spectrum of the sample in which Eu³⁺ concentration x = 0.16. It can be seen that the profile of the emission spectrum is different than those in other Eu³⁺ concentration-doped samples and the peak of ⁵D₀ → ⁷F₂ transition shifts from 612 to 613 nm. So we think the emission spectrum is an overlap of those in Ba₃Al₂O₆ and BaAl₂O₄ hosts.

Figure 6

Emission spectra of different Eu³⁺ concentration-doped Ba₃Al₂O₆ samples under 394 nm excitation. The inset shows the dependence of integral intensity on the Eu³⁺ concentration

Figure 7

Dependence of asymmetry factor R on the Eu³⁺ concentration under 394 nm excitation wavelength

The effect of charge compensators on the photoluminescence properties of Ba₃Al₂O₆:0.06Eu³⁺

The replacement of divalent Ba²⁺ ion by trivalent Eu³⁺ ion will produce defects in the crystal lattice, so Li⁺, Na⁺ and K⁺ are often introduced as charge compensator to reduce the distortion of local environment symmetries of optical centres caused by the defects and enhance the overall photoluminescence intensity [30]. Figure 8 shows the XRD patterns for Ba₃Al₂O₆:0.06Eu³⁺ calcined at 1450 °C and its charge compensated (with Li⁺, Na⁺, K⁺ ions) counterparts. The XRD patterns show that Ba₃Al₂O₆ is predominant in the samples. However, the impurity phase BaAl₂O₄ will increase in the charge compensator co-doped samples comparing with those without charge compensators.

Figure 8

The XRD patterns of Ba₃Al₂O₆:0.06Eu³⁺ samples doped with different charge compensators

Figure 9 shows the emission spectra of the samples under 394 nm excitation. The shapes of the emission spectra of all the samples are very similar, indicating that the introduction of charge compensators (Li⁺, Na⁺, K⁺ ions) does not change the sub-lattice structure around the luminescent centre of Eu³⁺ ions [32]. However, the emission intensities of ⁵D₀ → ⁷F₁ transition are significantly enhanced by the addition of charge compensators. In the Eu³⁺-doped Ba₃Al₂O₆ host, not all Eu³⁺ ions go into the lattice and occupy Ba²⁺ sites, instead impurity phases Eu₂O₃ or BaO may exist in the samples because of the charge imbalance caused by the substitution of divalent Ba²⁺ ion with trivalent Eu³⁺ ion. The existence of Eu₂O₃ or BaO could restrain the Ba₃Al₂O₆ grains growth during the sintering process, leading to the decrease in emission intensity [32]. The addition of charge compensator can help to incorporate Eu³⁺ ions into Ba²⁺ sites by compensating the different charges between Eu³⁺ and Ba²⁺ ions. So the emission intensity can be enhanced. Furthermore, the emission intensity depended on the type of charge compensator. The inset in Fig. 9 shows the dependence of integral intensity of ⁵D₀ → ⁷F₁ transition on the type of charge compensator. The emission intensity of the sample with Li⁺ as the charge compensator is higher than those of samples co-doped with Na⁺, Eu³⁺ and K⁺, Eu³⁺. The reason is that Li⁺ ions are easier to enter into the interstitial site of Ba₃Al₂O₆ crystal lattice than the others and introduces the smaller lattice distortions, because the ionic radius of Li⁺ (0.076 nm) is smaller than those of Na⁺ (0.102 nm) and K⁺ (0.138 nm) [5, 33]. However, the emission intensity of the sample co-doped with Na⁺, Eu³⁺ is lower than that co-doped with K⁺, Eu³⁺, which is contrary to the results obtained in ZnMoO₄ and CaBO₃Cl hosts [32, 33]. The reason is possible that the ionic radius of K⁺ is similar to that of Ba²⁺ (0.136 nm), so K⁺ ion is easier to substitute Ba²⁺ ion than Na⁺ ion.

Figure 9

The emission spectra of Ba₃Al₂O₆:0.06Eu³⁺ samples doped with different charge compensators under 394 nm excitation

Conclusions

Eu³⁺-doped Ba₃Al₂O₆ phosphors are synthesized by high-temperature solid-state method. The influence of calcinations temperature, Eu³⁺ concentration as well as charge compensators on the crystal structure, morphology and photoluminescence properties is investigated. The XRD analysis confirms that cubic phase Ba₃Al₂O₆ is predominant in the as-prepared samples. The crystal structure is not changed in the four samples calcined at different calcination temperatures and those co-doped with charge compensators. However, the impurity phase BaAl₂O₄ is obtained in the high Eu³⁺ concentration-doped samples. The morphology of the sample depends on the calcination temperature heavily. The particles are converted into nanorods when the calcination temperature exceeds 1350 °C. The photoluminescence spectra studies reveal a strong peak at 589 nm due to ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions, when excited at 394 nm. The emission shows an increase in intensity with increasing calcinations temperature from 1300 to 1450 °C. The optimum Eu³⁺ concentration is about 12 % for the sample calcined at 1450 °C. The emission intensity can be significantly enhanced by the addition of charge compensators. The sample doped with Li⁺ as charge compensator exhibits the strongest emission intensity. Therefore, Ba₃Al₂O₆:Eu³⁺ phosphors with charge compensator are potential candidates for UV-excited WLEDs.